Hello PeteDeal,
Fuhrmanns answer concerning your gun is perfect.It was built and nitro proofed 1937 in Ferlach and the gun no. is 1779.
The stamping 7,3 on the barrel foot means .284/7,21mm bullet diameter.
So the following cartridges are possible :
7x57R
7x65R
7x72R
As you already measured the shell lenght with 65,0mm , so it's 7x65R
regards Johann