Granted, many Super-Foxes were chambered for 2-3/4" shells but Supers were being ordered and factory chambered to 3-inch as early as 1923 even though (as Dave reported) the Western Super-X 3-inch/1-3/8 ounce Record load wasn't introduced until 1924-25. Possibly the "Fox memo" referred to by Bill was intended for owners who were handloading for these earliest 3-inch factory guns even though its wording doesn't say that. The blue colored memo/note is shown in Dave's earlier post here - and based on its small size - the context could have been as an insert placed inside boxes of 3-inch Record shell hulls to advise handloaders of the then-new factory 1-3/8 ounce Super-X's.

It appears that Fox was recommending 3" shells, to be used regardless of whether the user's gun was chambered for 3" shells or not. It seems like a "white collar" oversight.