The 16/C in the Diamond simply denotes it is chambered for a 16ga shell without actually specifying the length. This gun was proofed in the era where all guns receiving a Nitro proof had to have chamber length & diameter specified. As Pete stated this is 75mm (2.953") long or essentially a 3" chamber. The diameter was measured at the largest point or just inside the rim seat. The 18.9mm converts to .744" which is well within the range for a 16ga chamber. It would taper down some .015" to the forward end just ahead of the forcing cone. This situation fits the 1898-1924 date range Pete stated.

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