My Sauer und Sohn Suhl made 16/65 side by side is proof dated 2/29 (February 1929) and the serial number is 296XXX. The mark 6/31 is surely your proof date. To highlight such marks on blued barrels for photos, I use white chalk but crayon will also do.
The reason there's a W (Würgebohrung)only on the LH barrel is that it is the more heavily choked. The U (Untersuchung) is an inspection mark after proof firing. The S stands for shotgun (Schrotlauf) supplimentary proof. These marks with their Imperial Crowns and Eagles were only used by proof-houses in Germany between 1891 and 1939. Similarly the 16 in a circle, the nominal gauge. The 16 not in a circle is the actual bore diameter 22cm in front of the standing breech (according to the English proof system).
The word Nitro, however, was only used for shot barrels proofed at Suhl after 1912. So it could not have been made earlier than 1912 nor later than 1939.
SuS and the caveman are both Sauer und Sohn trademarks. Beware that, there is no "70" on the barrel flats and, unless it is instead stamped on the ejector, the shotgun will most likely have shorter (2,1/2"/65mm) chambers not suitable for 2,3/4" /70mm shells.
