I don't know that there's a lot of doubt.  It would seem that the um date of manufacture would be 1875 as that is when the change in the name to Lang & Son occured.  What is the address on the top rib 22 Cockspur?  I think the  tubes wear the "J.L." stamp, not J.C., a J. Lang being either the son-in-law of James Purdey or grandson of James Purdey.  Does it have the March 8th 1870 patent #687 Lang screw-grip action?  I wonder if there are worn proofmarks closer to the forend hanger?
Kind Regards,
Raimey
rse