I don't know about M880 markings specifically, or if they are done the same as other vehicles, but if they are maybe this will help. It applies to vehicles from the early 80s onward in the 3-colour camo or tan schemes. This doesn't come from official sources, just notes from vehicles and photos I have seen, and from the original markings on my Marine M813.
On most Marine MV's I have seen, the bumper markings are USMC on the drivers side and the 6 number VIN on the passenger side, front and rear. The USMC and 6 digit number are also on both sides of the hood near the rear hinge line. On my truck, the door number is also on the passenger side on the front bumper, under the vehicle number, and centered on the back panel of the truck under the tailgate. This particular marking is not consistent though, I have seen several other trucks that don't have the door numbers on the front and/or back at all. The door number also seems to wander in placement, on LAVs I have seen it centered on the rear door, above it, or on the passenger side next to the door, and on some trucks it is on the tailgate or under it.
The door number (in my case M21640) was explained to me by a Motor T Marine one time, but I don't know that I correctly recall what he said. I believe he said the M was for Marine, the 2 was for 2nd MarDiv, the next numbers were battalion and regiment (1/6, which makes sense, my truck came off Camp LeJeune and all those numbers are consistent with the 2nd Marine Division stationed there) and 40 was the vehicle number somehow. If I recall this incorrectly, I would be glad to be put right. Likewise if his explanation was wrong, I would like to hear about it.
All markings are in 2" stencils with green on black, black on green and brown.
Hope some of that helps you.
Cheers,
Rory
