I've been shooting the M1 a long time, thousands of rounds and I never got a M1 Thumb. It's easy to single load without getting your thumb. Just start the bullet in the chamber, hold the op rod back with the heal of your hand, push down on the follower assembly, ease the op rod forward until the bolt starts over the FA, and let her go.
Can't say the same for the M14/M1. It got me more then once. While single loading you have to put the round into the magazine and I am forever hitting the bolt release.
Never got a M16 thumb though.