raimey:
model numbers on robusts can show up in three different places;

clearly marked on the bottom metal, usually in conjunction with "Robust" (but if there are no manufrance identification marks accompanying the proofs, i would doubt that marking to be present)

stamped onto the bottom of the plume (rib) somewhere forward of the barrel flats

and on pretty late guns (?) marked on the top strap, concealed beneath the lever.

if it is marked 222, then according to charts found here in the french documents, it's a post-war gun - production from 1946 until 1963.

best regards,
tom

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland