It's my understanding there are two different lower patterns. The DPMS and Armalite. Both the DPMS and the Armalite have been around before the 1994 large capacity magazine ban. (assault weapons ban under Clinton) However, Armalite changed their lower pattern to use modified high capacity M-14 magazines. This is called the Gen II lower by Armalite.
I understand that Armalite, Noveski, Eagle and Aeroprecision (new AR10 lower) follow the Gen II pattern and several other makers follow the DPMS pattern. Unfortunately, Magpul does not make mags for the Gen II pattern. As far as I know, only Armalite and maybe Noveski is making the Gen II magazine. I have not heard of anyone else making them. kwg