Nial,
A coupla' hints are there, if you look for them. The buttplate says "Darne" and is the usual one in place on post war guns. Not definitive, of course, but, the monobloc tubes and safety in the breechblock, as opposed to the rocker in watertable, kinda' seal it as a Darne, not an F. Darne.
A photo of the barrel flats might tell us R13 or R15-I'd not bet R17, but, hard to tell without seeing the code on the flats and a matching serial number to the rest of the gun.
There are sliding breech guns out there with barrel serial numbers that don't match.
Chicanery happens.
Best,
Ted
