Given these are 40 years old one might expect a bit of problem.
Once per mag the 59 leaves a loose case in the port lying length ways on top of the mag and of course stops the slide.
It does this with factory mags with good springs, new Mec-Gar mags and other mags.
The extractor looks fine with no chips or damage. It has plenty of tension. It will hold a live round under the extractor while shaking the slide (old 1911 test).

The jam occurs mid magazine (about the seventh to tenth round).
Ammo is Federal 115 or 124 or Rem 115 FMJ.

I cannot understand what is causing this.