All of the Tamiya Half-Track kits are based on the M3 series of Half-Track, as they should be, and were originally released in 1975. Kit quality for that period of time being quite good, and despite their age, they are still very enjoyable kits to build. They all have their fair share of ejection pin marks to be filled and sanded smooth, but with a small amount of extra work, these kits will build up into excellent scale model replicas.
I have also built the M16 Half-Track version from this manufacturer, having completed this model kit back in April of 1985. Still looking good almost 17 years later too! Fit and alignment is generally quite good on these kits, but not quite up to today's standards.

Probably the most notable error with all of the Tamiya Half-Track kits is the incorrect drive and idler wheels and tracks. There are correct resin substitutes available for the wheels as after-market items, purchased separately, but I did not choose to go that route. I would however, highly recommend these after-market items as they add a tremendous amount of accuracy to the drive area of these vehicles. The tracks provided with the kit are correct as far as the "continuous rubber-band" track found on the real vehicle goes, however the guide horns are not correct. Also, to correct the inaccurate slack in the kit tracks, I had to positioned the drive wheels as far forward on the chassis as possible and the idler wheels as far to the rear as possible. After dry-fitting the tracks once again, I realized that there was still too much slack and decided to wrap the drive and idler wheels with .015" thick styrene plastic. This seemed to tighten up the track tension to a more acceptable/realistic level.

The Details and The Extras:
I'll let the pictures tell most of the story here. Basically all that you see in gray or white styrene was scratch-built. The rear bulkhead with stowage racks and the front right fender stowage came from the M3A2 kit. The tan styrene parts are from Academy and the Photo-Etch bits are from Verlinden and Eduard. All 'straps' were made from strips of drafting tape and the associated 'buckles' are made from tiny bits of styrene plastic. The canvas cover 'bows' are made from formed .015" diameter brass rod.
I opted to scratch-build the scaling ladders on both sides as I didn't care for the molded on ladders provided with the M3A2 H-T kit.
The mounting brackets for the mortar baseplate on the rear door were scratch-built.

The tarps in the rear stowage racks as well as the tarp hanging on the Fighting Compartment's wall were carved out of a self-hardening clay. 
Fine copper wire was used to link the radio handset to the radio. I 'wired up' the radio to the antenna, however further examination of my reference materials revealed that the manner in which I have shown this connection is not correct, but as usual, I discovered this "flaw" after the vehicle was painted....too late. 
The antenna is made from .015" diameter music wire.

The Decals:
Most of the decals used here are provided with the kit. Other decals came from my spare decal box. The small amount of stenciling used is from Archer Fine Transfers.

The Figure:

The Armor officer checking over what he has just signed for, is from the Tamiya "U.S. Infantry/Northwest European Theatre" figure set and is also molded in styrene plastic. The quality of this figure set being to the usual high standards that this manufacturer is well known for in recent years. He is basically built straight out of the box with some slight modification to his right hand. I have also added the helmet straps made from thin strips of drafting tape and Aber Photo-Etch 'buckles'. I also 'laced up' his gators with thin stretches of heated and stretched styrene. Decals used are from Super-Scale and Verlinden. He is 'pegged' to the base with a length of .020" diameter steel rod with a corresponding hole drilled into the 'concrete slab'.

The Base:
The wood is scrap oak and is cut to 10" x 6" with the edges routered with my Dremel Tool. The wood was then sanded and stained with a natural colored stain and then it was sealed with multiple coats of Polyurethane.

The 'groundwork' is constructed of Evergreen styrene with the top slab being .080" thick. I scribed the expansion cracks in with various scribing tools and a straight edge and then sanded smooth. The sections are scribed to 12' x 12'.

The paints used to paint the concrete surface are from the PollyS line of acrylic Railroad Colors, airbrushed on, sanded smooth. I then applied multiple coats of thinned oil washes using a variety of grays, black and Burnt Sienna. I then tapped a brush loaded with thinned black wash to simulate the oil stains.

The brass nameplate is 1/2" wide by .015" thick and is from the K&S line of brass products. I used fine steel wool to remove the tarnish and minor scratches. Letraset dry-transfer lettering was used for the description and once the lettering was down, I cut the brass to length by scribing (and scribing) a cut line with a #11 X-Acto blade. The rough edges were taken out with a fine metal file. I then applied a generous coat of Future Floor Wax to seal the nameplate and lettering. It is attached to the base with double-sided tape.

Painting, Weathering and Finishing:
PollyS/PollyScale, Model Master and Tamiya Acrylic Paints were used as well as a small amount of Enamel Paints from Testor. Winsor and Newton Oils thinned with Loew-Cornell Odorless Brush Cleaner and Thinner were used for the washes. Dry brushed entirely with Acrylics. Weathering was accomplished in part by using Berol Prismacolor art pencils. PollyScale Acrylic Flat Coat was then applied overall to the finished subject. Microscale "Kristal Kleer" was then applied to the headlights, taillights and dash board gages for that 'glass' look. Once all was completed, I glued both the vehicle and figure to the base with white-glue.

I have modeled this vehicle as one of the pre-delivery examples (#79), therefore there are no "WD" numbers applied here and it is not extensively weathered.

General info:
With the basic dissatisfaction of the M4 series of Mortar Motor Carriages, the Ordnance Department issued a requirement in 1943 to mate the M1, 81mm Mortar to the M3 series of Half-Tracked Personnel Carriers that were then currently in production. 

A prototype vehicle was produced in 1943 by the White Motor Company and designated as the T19. With the test results of the T19 proving out to be successful, the vehicle was standardized in July 1943 and designated as the M21 Mortar Motor Carriage. A full-scale production contract was then awarded with production commencing in January 1944. The contract to produce the M21 MMC continued until March 1944 when production was terminated due to the sufficient numbers of M4 MMC (572 vehicles) and M4A1 MMC (600 vehicles) already in service (both being based on the M2 Half-Track) at that late stage of World War II. A total of 110 M21 MMCs were produced.

Actual Wartime Deployment:

• US Forces, ETO:
  The 54th Armored Infantry Battalion, 10th Armored Division received at least three M21 Mortar Carriers in France during 1944. These vehicles participated in the relief of Bastogne, and remained in service with the 54th until July 1945 (Occupational) when the 10th AD was stationed at Garmish-Partenkirchen Germany.
  

• US Forces, PTO:
  The M21 was used in limited numbers (unknown) with the US Army in the Pacific Theater. Photographic evidence suggests that the M21 participated at least during the battle for Peleliu Island in 1944.
  

• Free French Forces (FFL):
  Fifty-four M21 vehicles were delivered to the French Forces via the Lend-Lease Act, serving with at least one armored unit, the 2ème Division Blindée, or Lecler Division.

MMCs did not see any wide spread service after WWII, other than limited Occupational Duties, having been phased-out shortly after the end of hostilities. Israel being the exception and then only in a very modified (M3 with 120mm Mortar) form.

The M21 MMC was a much more capable vehicle than the earlier M4 series of MMC, with the basic differences being:

• Longer M3 Half-Track body with the M21 verses the shorter M2 Half-Track body utilized with the M4 series of MMC.

• Forward firing arc of the main weapon on the M21, vs. the rearward firing arc with the M4 series. The Mortar when fired from within the M21 also had a traversing capability of +/- 30° whereas in the M4 series, the weapon's traverse was limited to the minimal traversing gear mounted on the weapon itself.

• Reinforced flooring on the M21 to accommodate the constant recoil forces of the mortar that permitted sustained main weapon firing from within the vehicle. In the M4, the main weapon was to be fired from within the vehicle in cases of emergency only. The M4A1 did have reinforced flooring which allowed the weapon to be fired from within, however, the weapon still fired over the rearward arc. (Some M4A1 crews did field-modify their mounts to enable the weapon to fire forwards despite official policy and tactics.)

• Increased 81mm ready round stowage. M21 = 60, M4A1 = 48.

• Pedestal mounted .50 caliber M2 Heavy Machine Gun in the M21 vs. the M1919 .30 caliber Light Machine Gun mounted on a 360° skate rail in the M4 series.

• Increased internal and external stowage and Fighting Compartment crew space in the M21.

References:
"M3 Half-Track in Action", Armor No.34, Squadron/Signal Publications.
"M3 Infantry Half-Track, 1940-1973", Steven J. Zaloga and Peter Sarson

References on the World Wide Web:
Old Timer Picture Gallery

Additional Resources:
"US Half-Tracks in Combat, 1941-1945", Steven J. Zaloga
"Half-Track, A History of American Semi-Tracked Vehicles", R. P. Hunnicutt
"Half-Track", Janus Ledwoch
"2ème DB, Album Memorial", Alayn Eymard (Editions Heimdal, France)

Mr. Herve "Charby" Charbonneau - Quebec
Mr. Michael H. Pruett - United States
Mr. Frank Domalski - United States
Mr. Dave Lomas - United States
Mr. Robert Sargood - United States
Mr. Tom Hogan - United States
Mr. Roberto Flores -Spain
Mr. Tom Bebbington - United Kingdom