The stepped Alt key from a Model F AT board fits nicely on an M, but the stepped Ctrl key is too big. Being AT's have only one Alt key, you'd need two AT board to replace the both the keycaps on an M.
Edit: oops, should have read the OP better, and the pictures in the previous post didn't show up when I first read it. Yep, if you're replacing the Caps Lock key with a stepped Ctrl from an AT, it should be fine.