John: I believe Post-1913 Field grade guns could be ordered on a Regular frame. Featherweight frames (introduced in 1909) have two sizes: 12g and 16/20g Regular frame guns had three dimensions: Syracuse & Fulton Pre-1898, Post-1898 12g, and Post 1898 16 & 20g. Only 6 experimental 'C' frame guns were produced, all in 1929. The Long Range Wild Fowl guns were made using the R frame, but were slightly wider and heavier. The engineering drawings are on p. 372 of John Houchins' L.C. Smith "The Legend Lives"
Left to right: 12g R frame 2 3/4" Specialty, 12g R frame 3" Wild Fowl Field, 12g FW frame 2 3/4" Field