The frames were gauge specific.
No they weren't. The 16- and 20-gauge Model 31s were all built on the same size frames in the 5xxxxx serial number range. The Remington Model 11 and "The Sportsman" were built on specific 12-, 16- and 20-gauge frames.
My two Remington Model 31LA "Standard" Grades a 12-gauge with 30-inch full solid rib barrel and a 20-gauge with 26-inch Cutts barrel are both just marked Model 31, but I have seen pictures of the Aeromet receiver guns marked Model 31L. Never had one in my hands.
