A couple years back I saw something similar for sale at the local-to-me Cabelas: a French 12ga 65mm sxs, with the usual St. Etienne proofs, with German proof marks for a date of 741. That gun also had the SuS (Sauer und Sohn) monogram as well as the Nazi Waffenamt eagle stamped into the buttstock and the watertable. Best I could figure was it was either a German soldier's bringback from France or looting by the German government, in either case having to be reproofed in Germany before it could be used or sold. And then a GI bringback.
It was a solid gun but, like just about every other French double, light. If I were in your shoes, advising the owner, I would pass along to him that he - and the gun - would be a lot happier if he were to limit himself to 2 1/2 inch shells and, as you note, lower pressures. If the gun is in good shape I wouldn't be as worried about pressure (esp. from promo loads) as I would be about the recoil. This, especially because using 2 3/4 shells in a 2 1/2 chamber will enhance recoil. With a light gun, the recoil effects on shooter and gun are even greater.
