Note the 16/C in diamond means it has a 16ga Chamber (C). When proofed the actual bore diameters of this gun were 16.9mm (.665")
& 17.1" (.673"). The bbls were choked to some unspecified amount.
As stated by Larry the 16-C in diamond means oit was proofed no later than 1924. At this point the 16-65 or 70 in the Elongated C replaced the diamond mark. In this latter case the 65 etc following the gauge number is the length of the chamber in MMs. 65 = to about a 2 9/16" chamber & 70 a 2 3/4" one.

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