I believe, but am not completely sure, that all early Remington 870s wiere built on a 12 gauge frame. The 20, first built on a 12 frame, later came out in a lightweight on it's own frame size. Eventually all 20s were transitioned to the newer, smaller frame. To the best of my knowledge, all 16 gauge 870s were on the 12 gauge frame size, and the one that belongs to a friend of mine, though a nice shotgun, is only trimmer than a 12 in the barrel and magazine tube.
I have a 1951 20 gauge 870 and a 1953 16 gauge 870, both in mint condition. Both are on 12 gauge frames and I believe the serial numbers have an "X" prefix. I much prefer them over the later 870s with the gigantic oversize forearms no matter what frame size the later guns are based on.
