Bernie: You have what appears to be a H&H Royal Double Rifle in .500 Black Powder Express. The gun has a Jones underlever (quite typical for an 1880's gun) The hinged express sights are regulated for various distances (I don't know what the hinged sight on the muzzle is). The tools, turnscrew, etc. are compatible with the gun. There would have been a trunk case with a charge card with the gun originally.
You can order a letter from H&H (I think for about 30 GB Pounds) that will give you some history & the original measurements along with the caliber of the gun. Their address is:
Holland and Holland
33 Bruton Street
LONDON, UK
W1J 6HH
Tel: 011 44 20 7499 4411
FAX: 011 44 20 7408 7962
E-mail: gunroomuk@hollandandholland.com
I hope this helps.
Best Regards, George
