Just curious have you seen this before?
Description in upcoming Holts auction:
AN ANOMALOUS JOHN RIGBY & CO. .577/.500 (NO.2) BP EXPRESS TOPLEVER / ROTARY-UNDERLEVER DOUBLE-BARRELLED HAMMER RIFLE, serial no. '14188' / '999', circa 1879, 26in. Rigby trademarked black powder only barrels with file cut rib engraved on gothic script 'JOHN RIGBY & CO. DUBLIN & LONDON', with open sights and folding leaf sight, missing extractor screw, loop rebrazed and some distortion to under rib, carved percussion fences, non-rebounding back-action locks with repaired ornate bolted dolphin hammers (right hammer fractured at previous repair to spur), action flats with Rigby trademark, original 5 digit serial number defaced and new number '999' stamped (also to barrels and fore-end iron), Jones patent screw-grip patent rotary underlever and secondary diminutive toplever (operating a third sliding bolt engaging with rear lump), requiring the underlever to be operated to approx. two thirds throw and then the toplever to release the final third of the underlever operation, action bar heavily polished with some later naive border engraving, engraving to lockplates, underlever and hammers worn, replacement triggerguard with number 14188 engraved with an amateur hand, fore-end with identified Rigby's Patent grip-catch release, the iron (with repairs) with original 5 digit serial number defaced and later '999' stamped, some repairs to wood,14 5/8in. replacement pistolgrip stock with pistolgrip-cap, sling swivels and 3/8in. recoil pad, weight 9lb. 2oz.

577 Rigby