just interesting infor for some - i haven't seen t=any of these for years, but back to times closer to ww2 you would find m1 clips with the back end ridges all but ground off. this was done for faster reloading. a regular clip will hole the action open, but if you grind off the back it weakened the clip enough so when you jammed it into the rifle the bolt would close without touching it. other thig was it was common to find rifles that had the right side of the stock beat up. this was caused by loading a clip and slamming the end onto the side of the stock to make sure all the rounds were seated fully
just some old info some might find interesting

I was just out shooting an M1 this morning. I have seen stocks with dents in the stocks from soldiers that were taught to jam the clips into the wood to make sure all the rounds were seated into the clip. This was NOT taught, officially, to US soldiers, but is something that comes from the imports. As for clips being ground off; that accomplishes nothing. If you insert a full clip, the bolt will be released. A regular clip will not hold the bolt open if it is full. I am not sure where you heard about grinding clips but the clip is not what activates the bolt release; the ammo in the clip does that. Putting an empty clip into an M1 does nothing; in fact, it wants to be ejected and the bolt is already held open by the bolt catch on the operating rod.