Maybe try a 20 for the Sav-64 ? The 'hole part' of the 'keyhole' shape might be ground down a bit to fit ? OR "hot-knife'd" to fit ? Just S-ballin' on this, I have a couple 64s but Not the 20-round mag. They cost about $60 + and I don't really 'need one. I've got about 10-12 regular mags for 'em. And I don't know how 'thin' the MKII mags are.
I'd measure the MKII mags then go to the LGS to 'measure and ponder' on the S-64 mag.
Won't work. 64 mags angle back, MK2 angle forward. Then MK2 has lip on the edge of the mag as a guide over the mag release.