Sounds like someone switched out a magazine, happens a lot.
Anyway, the USGI or Colt 20 round mags were some of the best ever made, I have several and used to buy them for $4 and $5 even during the ban years at pawn shops and gun shows. A few years ago I bought a dozen new Colt mags for $10 a piece, switched followers and put them up. I mostly use 20 round Pmags now but the Colt mags will always be my favorite.
