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.
