It is an R13 that was built pre WW1. It was originally proofed at 65mm, but, as Gene pointed out, (Thanks, Argo) there was perhaps a decade just prior to the first world war when they were marked 6.5 for this identicle chamber length. It has scalloped breech balls, a feature not seen much after the 1920s. It is somewhat odd in that it has that feature, is a later, 1909 patent Darne R model, is a 28 gauge, and does not appear to have monobloc barrels.
Someone, perhaps in Europe, perhaps here, marked the chamber length as a conventional 70mm. I do not see the French marking for reproof, but, the gun had pretty serious proof applied the first time, so, I sure wouldn't let that fact alone be a deal breaker for me. They need to be measured to see just what you have.
It would be a wonderful grouse and woodcock gun, assuming it fits the user. The wrist cannot be bent, so, don't try.
Good luck.
Best,
Ted
