Some pictures:

Sliding breech gun with actual side locks.

Sliding breech gun with disc set strikers.
Bizarre amount of cast on original stock:

Carl and his stocker have been busy:

Safety side of gun:

Top of gun. Says Francisque Darne, differentiating from the typical Regis Darne product. Francisque was Regis Darnes eldest son, and opened up his own shop in 1910. He died in 1917, but, his company soldiered on into the mid 1950s with different owners.

Carl, a good, clear photo of the barrel flats would be most helpful in nailing down some more details about a very unusual sliding breech gun.
See what you can do. Ill look through my catalogs to see what I come up with.
Best,
Ted