Generally speaking, though as always there may be exceptions, the D = 70/20.6 was used from 1892 - 1923 only on those guns which had been submitted for "Voluntary" smokeless powder proof, though you do not indicate a PV mark. This mark indicated a chamber length of 70mm (2 3/4") having a diameter of 20.6mm (.811") just forward of rim seat. This of course varied as to actual chamber dimensions.


Miller/TN
I Didn't Say Everything I Said, Yogi Berra