Most of the "A" (reportedly the "A" started as "Alteration") versions of the M4 really involve the engine. The M4A3 had the Ford model GAA 500hp 8-cylinder, liquid cooled engine. The M4A3 was the preferred tank in the US Army for the second half of the war (the M4/M4A1 were preferred in the first half). The M4A3 was basically reserved for US use, the British got about seven, I believe, for testing only. Later in the war the French started to get a few M4A3s with 76mm guns, but again in very limited numbers. Strangely, the earlier M4A3 (like FoV produces) didn't get into combat in significant numbers until 1945, AFTER the later versions had already seen months of use. They had been training vehicles in the US and were sent over as replacement tanks due to shortages created by the Battle of the Bulge.
99% of the flail tanks built were British, and their most common tank was the Sherman V (M4A4). The Flails were often labelled as Sherman Crab". The M4A4 tank had the Chrysler Multibank engine that was actually five six-cylinder automobile engines working on a common output shaft (a 30-cylinder enhine!). Anyway, the M4A4 had a slighly longer hull and different rear deck details that the M4A3. For instance, most Shermans used 79 track blocks per side, but the M4A4 required 83.
The Marines and Seabees built a flail tank from an M4A2 (twin GM diesels) in the Pacific. There MIGHT have one M4A3 flail built in Korea, I'd have to haul out my books and check. The US Army in Europe deployed two units with mine rollers on M4 or M4A1 types, and the British did loan the use of Sherman Crabs to support some US units, especially in the Roer River campaign.
By the way the "E8" in M4A3E8 referes to the wider tracked HVSS suspension. Look at the tracks on Andy's DD27 and DD45 Shermans. DD27 has the HVSS suspension while DD45 has the "regular" VVSS type. As a former Armor man you can appreciate the virtues of the wider track providing better floatation.
Anyway, while critical of the "M4A3" reference, I DO think that a Sherman V "Crab" would make an impressive model. It would be great to be able to have the chains hanging loose, and maybe the drum could be adjustable from the travelling position to the flailing position. Not only the flail gear up front, but there were bins on the back to hold lane markers, plus British markings and stowage, it would be a very interesting Sherman variant in anybody's collection.
Gary