Prevent side to side movement apparently.
Same concept as sideclips?
That was my thought as well. Nothing else would make sense.
I don’t think so.
The lug clearly penetrates the action flats, telling us it is free to move up and down, even if it is only a bit. That size lug, in that size cut in the barrel would not resist side to side movement very well. My guess, and that is what it is, looking at only a picture, is, it keeps the gun from firing unless the opening lever is completely closed.
Best,
Ted