Interesting question that I never thought of before. I have lined up some M781's from my collection starting with one on the left that I cannot ready the date on, but it is marked XM781 so it is early. The next ones are 1980, 1981, 1983, and 2000 respectively. As you can see, my 1983 one has a plastic rotating band, but it is surrounded by metal ones. This makes me wonder two things. First, since the lot number, etc. is on the M212 cartridge case, is that the date of the case and not the complete round. Or, is there a "mod" to the projectile specs that allows the use of either type of rotating band so the contractor can choose either depending on cost and/or production capabilities. According to my documentation I think the round was fielded in 1976 and of course, it had a metal rotating band. The only other projectiles I know of with plastic rotating bands are the M1001 Canister and the XM1006 less than lethal, but there may well be others I can't think of at the moment. Hopefully someone has some firm information.
