That confusion extended into the model marked guns too. Over the years, I have seen many square-butt Model 30s, and a few round-butt Model 31s. Seems there were pictures of similarly marked guns in one of the SCSWs. I suppose this is another example of S&W's well known tendency to, on occasion, manufacture by convenience and economy.

Most of the square-butt 30s I have seen have 2-inch barrels, and most of the round-butt 31s seem to have 4-inch barrels, so maybe those running the show at S&W at the time equated the barrel length with the model as much, or more so, as they did the shape of the frame... ? I have never heard any real authorities bother to explain this, so my explanation is pure speculation.