2-holer is correct. The changeover to 2 3/4" as standard was not done all at once by all American gunmakers. And as a matter of fact, it wasn't even done all at once by individual gunmakers in all gauges. As of 1940, per my Shooters Bible, there were still far more shotshells available in the old, shorter lengths for the 12, 16 and 20 than there were in the "new" (and by then nearly universally standard, at least in the US) 2 3/4" length.