You are correct; 1904-1925 (there is no chamber length marked) and the 20 over C indicates 2 1/2" chambers. 2 3/4" chambers would be 20 over LC.
21 = bore .605 - .614" (.615" would be marked 20)
The standard load for 2 1/2” cases was 33 gr. “Schultze” = 2.36 Dr. Eq. (about 2 3/8) with 13/16 oz. shot.
7/8 oz with 2 1/2 Dr.Eq. (1210 fps) with Nitro loads of that era would run about 10,000 psi
As Matt warned, CIP allows for both 65 and 70 mm 20g max. service of 12,038 psi; with MSIP 950 BAR or 13,779 psi
