I had a very good day:

Found that the GSG 22 mags work great in the Kart conversion (better than the original mags).

The GSG also work in the Marvel but probably shouldn't use them because of the hold open.

The cheap plastic Advantage mags work great in the Pachmayr conversion.

Both units work with the Metalform mags which are 4+ times the price of the Advantage if you can get them.