Swift 1;
Have you measured the chamber length on this gun. That 75/20.6 is chamber dimensions. Normal metric/inch conversions show a 65mm as 2 9/16"; 67mmm = 2 5/8"; 70mm = 2 3/4", 73mm = 2 7/8" & 76mm = 3". 75mm converts to 2.95" or less than a 1/16" shy of 3".
The 20.6 is the diameter of the chamber just ahead of the rim seat. This converts to .811" which is a correct dimension for a 12ga chamber.
I forget the exact dates now but this marking became mandatory on smokeless powder proofed guns at a quite early date & continued in use until 1924 as I recall. Also I do not recall at what point the date codes came into use but this gun likely predates them.
