One more thing we taught that can be done as a "2nd layer" preventive measure to avoid an M1 thumb is to be sure the thumb is pointed toward the muzzle of the rifle, not downward, as the clip is pressed downward. If thumb pointed toward muzzle and you fail to control the bolt handle, there is a better chance the bolt will push the thumb up and out of the way as it slams forward.
Regards,
hps