Infomation for future reference, on a german proofed gun, a 16 in a circle means it was chambered for the standard length shells, which were 65mm( 2 1/2-2 9/16").If it were chambered for a longer shell, it would be required to show the chamber length next to the circle.For 2 3/4" chamber, the mark would be 70(ie 70mm).If it were rechambered later, a 70 or 2 3/4" might have been stamped somewhere else on the flat, or even the extractor.
Mike