early colts, 1912-13 was having a cracking problem on mags. Came up with a fix on about 25K mags. Slice and put hole in rear of mag [as pictured] Because they are rare, Mucho dinero per mag. Even rarer are the full base plate where sides of mag end at plate and base plate is under side. I have seen them at 1K per mag. SA mags with loop 325 all day long and w/o loop with folded lips 500+ all day long. Will post some pics of my meager collection after I shovel the latest snow!
The cracking occurred on the back of the magazines in the area indicated in the photo. Colt was having a hard time getting their tempering correct on the magazines, and the cutout in the "Keyhole" magazine gave more spring to this area prone to cracking because of the sharp angle. This area was ground by hand, and some magazines had a sharper less rounded corner.