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.
Are you sure about this? I was told my 16 ga. Robust No. 214 s/n 179*** was a pre-war model, yet is is marked "850 kilos."
Now I'm wondering...
GWP, see WC's earlier post on this subject. Could be, as he says, that Manufrance marked it that way. But I don't believe any of the European proofhouses were stamping guns with proof pressure in kg (or bars) as a matter of course until after WWII. The Brits didn't start doing that until the 1980's, but they were about the last ones to change.