Bob Naess seems to know pretty good numbers on rarer guns like these because he deals with working on them. Not sure there is any way to get the data out of ATF. I think there might be more ZB26 guns than you think. They are pretty regular at the RIA and Murphy's big MG auctions. ZB30 are much rarer but if I would guess I would say the ZB26 might be 1/5 the number of US BARs not that gives any numbers.
No.
The M3A1 has a smaller rear plate that is rectangular. They went to the smaller plate so the bolt/recoil assembly could be removed from the receiver without removing the ejector housing.
The M3 one is round and requires the ejector housing to be removed from the receiver for the bolt/recoil assembly to be removed/installed into the receiver.
You can use either style in either gun with the ejector housing restriction. The M3 one is considered better because it spreads the recoil load over a larger area of the receiver rear.
I bought my first MG, RDIAS, from him back in 1993. He also did some threading for me. He had a few one offs of just about every type of conversion there was for C3 weapons.
Always helpful to me and remembered him always being at the Pasadena gun show.
May he rest in piece.
The last one is a Stoner sight adjustment tool. Are you interested in selling it?
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