J.P. Sauer proof mark help

[Karl]

I have a J.P. Sauer & Sohn Model XXIV drilling. The 12 gauge shotgun chambers are 2 3/4" per measurement. The gun was made circa 1894-1895. Owning to the age I'm surprised that the chambers are 2 3/4" and would have expected it to be 2 1/2". Is there a way to determine the original chamber size from the proof marks; photo is attached. What does 13/1 mean (clear on left barrel)? Thanks.

[Axel E]

The circled 12 indicates the nominal caliber of the barrel, that is the cartridge to be used. As there is no chamber length given, like 70 mm for 2 ?” shells, the gun was proofed for the old standard 65 mm = 2 ?” case length. To this day the 65 mm length is the only one that needs not to be marked, neither with the proofmarks on the gun nor on the individual cartridges. If your gun has 2 ?” chambers now, those were lengthened sometime outside European proof laws.
13/1 is a gauge number for the actual bore diameter as measured at the proofhouse with plug gages. Three such gauge numbers are within modern 12 gauge specs: 13/1, 12, 12/1. So your barrels were on the tight side at time of proof.

[Karl]

Thanks very much for a clear explanation. I have several 19th century cartridge shotguns and fire only 2 1/2" shells. I've fired this gun with 2 1/2" shells. The gun is in excellent condition and the bores are bright and shiny. Are they any standard references on German proof marks?

[mike ford]

Karl,
Yes, there are several references that tell what the primary marks mean. The reference I use the most is an article in the 1975 Gun Digest. This article gives some history and a pretty clear explanation, and has tables with the explanations. Be sure to read the footnotes also.
Mike