Thanks for posting the pic 9.3. The text of the ad is below. Not a lot of information but it does provide the barrel length and weight. The barrel near the breech is stamped ".577 Express". Looking at the proofs I would guess it's a .577 BPE.

Description:
English 577 Express Big Bore Rifle. Caliber is 577 23/4" Express. "W.W. Greener" 68 Haymarket St., London Address. First introduced in the 1880's in black powder form and later as the 577 "Nitro" Express form, both in 23/4" and 3" versions. Bullet weights range from 520 grains up to 650 grains in the 23/4" Express form. The "Nitro" for black powder loading was 75 grains of cordite smokeless powder and a 650 grain bullet. Built using a "Braendlin" Armory Co. "Martini" patented action. This company supplied these actions to the English gun trade for some time and was acquired by "W. W. Greener" in the later 1890's. Rifle weights approximately 9 1/4 lbs. and has a 28 3/4" barrel. The .585" bore is in excellent condition as is the metal and wood. As they say "Used but well cared for". This extremely rare rifle will make a wonderful shooter with the 75 grain "Nitro" for black powder load, or modern handload equivalent load and of course reduced loads. See the pictures for barrel "Markings", address , caliber, "Proofs" and any other details you may wish to know. Own this piece of Big Bore History for a "fraction" of the side by side Double rifle price! ASKING $5,999.