This is probably a Howdah rifle from India

by R B Rodda, Calcutta for the Duke of Edinburgh 1880ish

in .577/500 No.2

barrels are 19"

weight 8 1/2 lbs

A very handy rifle

The only reference in any shooting books (of the time)I have seen for either Howdah pistol or rifle was a 12 bore with round ball in one barrel and SG (0 Buck) in the other kept in the Howdah

A more detailed post is here somewhere

TH44