Well, I was able to use a Dremel rotary tool to cut out the ridges. I then used some course-grained sand paper to smooth away any burrs on the inside and outside. The magazine now holds 14 rounds but I haven't yet tested its feeding performance at the range.
Beretta Px4 .40 SW; Remington 870 12-ga; Saiga .223; .22LR Rifle
"If we wish to encourage a society where citizens come to the aid of neighbors in distress, we must not strip them of the actual power to do something about crime." ~ Murray Rothbard, For a New Liberty