What's the second cut for? Is it compatible with a 1911 catch? thx

The Sig P220 magazine button catch has a "[" shaped catch which holds both sides of the thin bar between the magazine cutout holes. This not only keeps the magazine from dropping out, but also keeps the mag from going to far in. Is it needed? Who knows, but that's the way the P220 mag catch has always been. Can also be a drawback because the mag catch can catch on the lower hole a little without fully grabbing the thin bar if you just lightly push it the mag in and don't seat it well. When this happens, the mag catch can sometimes hold the mag in temporarily, but not securely or well enough to be shot, as the mag will drop out.

With single cutout mags like on a 1911 or others, usually there is a single mag catch protuberance that sticks into a cutout to hold the mag in.