The 16 in a circle indicates 65mm (2 9/16")chamber length. If it were 2 3/4" it would show 16/70. Most German guns of that vintage have 65mm chambers. It is good to check the chambers with a gauge to see if the chambers have been lengthened.
Cheers,
Roger
The number in a circle is simply an indication of the gauge. I have a Sauer from the 1930's with 20 in a circle, and it's factory 2 3/4"--so indicated by "2 3/4 shells" stamped across the face of the extractors, and verified by Griffin & Howe records. The 16/1 on this gun indicates a slightly oversize bore from the factory: .669 rather than the 16ga standard .662.
