Pete M provided an excellent history of Lyon & Lyon of Calcutta, see earlier post.
The gun that I inspected had Birmingham proof marks. As I recall the proof diameter measured 9 inches from the breech was within proof tolerances,[ My memories is not perfect but as I recall the proof mark stamped on the barrels was 13/1 and based on this mark the diameter of the barrels should and did measure between .720 -.728 diameter]
The gun that I inspected was of medium quality, nice stock and case. Did not record any details because I was "put-off" because of the lengthened chambers.
If this is indeed your gun I would have a competent gunsmith inspect and if feasible conduct a proof test for possible use with low pressure shells.