Greener577,
Your nice Sauer was proofed in Feb. 1930(2/30), so it could very well be a GI bring back from WW2. The 12 in a circle means it has a standard, for the time, 12ga chamber. The chambers will likely be around 2 5/8"( often called 2 1/2"), instead of todays standard of 2 3/4". The 13/1 is the bore diameter ahead of the chambers, expressed in gauge measurement. This works out to be about 18.26mm. The crown S means it was proofed for shot. The crown W means the barrels are choked, but the amount of constriction is not shown. The crown U is the mark for a View Proof, which is a detailed inspection after undergoing definitive proof, including verifying dimensions. The "Nitro" in block letters means it was nitro proofed in Suhl, but since Sauer was located in Suhl this would be expected anyway.
Mike
