Those last photos must show a post-WWII Robust. I don't believe the 850 Kilos mark would have been used before the war. According to Kennett, the "normal" mark was used from 1924-62. The choke-demi choke marks are what I've seen most often (when choke is marked at all on French guns) for M/F.

Expect there might be a Manufrance SN/date list somewhere, but I've never seen one.