M is the series of truck, it came about after ww2, to standardize the truck inventory, after ww2 there were 3/4 tons, 1 tons, 2.5 tons 6 tons 7 tons 10 and 12 tons any many manufacturers, this was a night mare in parts and maintenance, all m series share many parts and like a hot wire to a light will be the same letter/number from a jeep to a tank,same gauges also are some examples, so m is a series , and to date the most produced and longest period of production of any known to me for sure, these trucks rock to say the least, that is what M stands for , X was an experimental stage I do believe, it did not change after the trucks came off the production line the Xs staid Xs, but when mast produced they were not x,s anymore, because they were ready for and in production, most of the time they were the same truck, but not every time. , also for the most part an M series could be produced from any manufacture and be the same vehicle, now like everything army there were exceptions like some motors, and other cool stuff that is important to a hobbiest, all the best
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oh a winch model will simply have W/W meaning with winch on her, many winches were or could have been put on after, so the W/W may not be there that would just mean out of the factory it did not have one. this is excpecially true in the hobby . field tested is not applicable in my opinion, it x... means before excepted/standardized and before ready for production,