Prototype Configuration

Interim Configuration

Production Configuration

This detail from patent number 1,348,733 actually shows the lock being round and the button with a half-moon cutout.  Probably just a sloppy error on the part of the draftsman..

These two detail drawings from patent number 1,397,109 show the workings of the magazine lock and button.  By this time (1921 patent) the groove had been incorporated into the design of the lock.  You can see why it was necessary in the right-hand drawing, because in order to remove the lock, you must move the bottom portion of it toward the rear of the gun, which is nearly impossible without a way to insert something between the magazine lock and the magazine lock follower (or release button, as I call it).