Marakai, Thanks for the clarification on your beautiful rifle! This may be a 577 SNIDER that was rechambered to 20ga-577 2 1/2" because the 20 bore cases were easier to get in India, or simply a special order by some one from the maker. But the Maharajas of india, had many doubles made on 12, and 20 bore brass necked down. The 577 REWA is an example of this special order from the Maraja of REWA!

No mine is not British, but German! It is a V. Haffner made in Cassel (old spelling, the new spelling is with a "K"), Germany sometime around 1890. It has beautiful Cercasion Walnut wood, checkered at 28 lines per inch, the butt stock is straight, but has a buffalo horn pistol grip, and butt plate. Like your A-H it has a jones under lever, and minimal engraveing, and is case colored.

I would love to own the rifle you have there, as I collect double rifles, or in fact, any firearm with more than one barrel! Both your's and mine are rare chamberings. The 58 Berdan musket, and arms so chambered are around, but even the cartridges for the 58 Berdan carbine version are rare, and arms so chambered are VERY rare in any condition. I would say the 20/577 1/2" is even more rare, since in all my 66 yrs I've never seen one of the cartridges, or a rifle for the cartridge. You have a treasure, my friend, treat it well!