So, I got some circle ones, a person would just need the connector, right?
Basically correct but a little more to it than that. Floor plates are different but definately not a big hurdle using blank plates. Spring steel shouldn't be too hard to come by. Rivets are easy enough to find. Catch and post may be a little tough for exactness. Only know of two people who have authentic pieces and getting dimensions for small parts hasn't been very successful to date.
Can't really see anyone reproducing them as they are pretty worthless in a real world situation. Definately retro and very rare for authentic but very small market also. One circle 1 mag has been on GB for a few weeks now at $50 with no takers but it isn't from a piggy back set up either. Guessing authentic piggy back mags with correct floor plate are worth $100ish ea in todays market. Find some one with more coins than brains and they could easily bring asking price though. What's $500 to someone with real coins? Finding someone with real coins that wants a piggy back 20 rd mag set up is the test.
Thanks for heads up. More pics to look at if nothing else.