I added some Wolff 17 pound hammer springs to one of my P64s and took it out for a function test. I experienced several stovepipe jams with one of my magazines, but the other 3 magazines worked perfectly. More importantly, in the P64 the hammer spring is also the magazine catch release spring. Switching our the hammer spring to a lighter one allowed the magazine catch to flex under recoil. The end result as shown in the video below was the last round of each magazine was accompanied by the magazine dropping free of the gun and the slide not locking open when the last shot was fired.
I will obtain some 18 or 19 pound springs and see if that solves the problem.