We've covered this before (so do some searches and see if you can find a more concise set of answers...)... But, off the top of my head:
M101: Five lug rims (Budd, 6.50Hx16, same as on the M37 familly of 3/4 ton trucks), 900x16 tires, brake handles have the "squeeze" trigger release and are mounted so that the move in an arc paralell to the front bed panel. Parking brakes only.
M101A1: Same as the M101 except the parking brake handles are the flip type and move in an arc perpendicular to the front bed panel.
M101A2: Surge brakes in addition to the mechancal parking brakes, hubs and rims/tires changed to match those on the Chevy CUCV's.
M101A3: Same as the M101A2 except the hubs and rims/tires changed to match the HMMWV.
Lights have been changed over the years and followed the arrangement that was on the designated towing vehicle at the time, so they have evolved from the little "cat eye" round lenses to the larger composit units and have gone from having only one service stop/tail light to two, ect... Lighting is an easy upgrade, so its commom to find a M101 with composit lights...
Not sure when the rear prop leg became standard. I've had M101's with one almost as often as I've had them without (although I have to admit that those that I've had with them seem to have been used as generator trailers...), just about every M101A1 that I've had or looked at had one... I haven't looked at M101A2's and M101A3's enough to make a definitive comment, but my guess would be that they would have one... They can be bolted on or removed quite easily, so retrofitting a trailer that does not have one is easilly done, and is very handy to have if you like to jump in the trailer when it is not hitched to a truck as it prevents it from tipping back on you (and dumping out your camping equipment for instance...).
I have photos of M101 and M101A1 on my website at:
Photos of my 1960 M101 3/4 ton trailer
Photos of a M101A1 3/4 ton trailer
Photos 115-119 on the 2005 East Coast Convoy page on my website show a M101A2 trailer:
East Coast Convoy and East Coast Rally
Don't have any photos of a M101A3 on my website currently...
Both my M101 and M101A1 will dump if you pop the pins, unhook the wiring from the tub, and (as FrankUSMC reminds us) chain the tailgate up so you don't bend it... Not something that is mentioned in the TM, but something that can, and is, done...

I suspect that dumping a M101A2 or M101A3 would be just as easy since they are essentially the same trailers except for the axles (hub/rim/tires)...
Good luck.