Thisis a proprietary Webley boxlock with nitro proof and 'normal chambers', which means 2 1/2" (not 2 3/4").
The gun was proofed for 1 1/4 oz and was therefore likely intended as a 'fowler. Live pigeon guns and 'fowlers tended to be proofed for this load (rather than 1 1/8 oz)and were built heavier (a game gun would be 6 1/2 - 7lbs) with semi-pistol grips.
This does not mean you can put 2 3/4" shells with 34g loads in it now!
It should handle a 67mm shell with 34g, like Eley Maximum OK.Otherwise stick to a 65mm 30g load for most of your shooting - it will handle those fine. Check current bore size and wall thickness first as it may well have been lapped since originally proofed.
