The gun is an Alex Henry and it was marked on the bottom of the receiver conversion by Alex Henry. Conversion from what to what I do not know. It was suggested at the auction it was a 500 BPE. This is the description of the gun "Alexander Henry ( 12 St. Andrews St., Edinburgh , Patent # 1071 ) Side By Side Hammer .500 BPE ? Cal Double Rifle w/29" bbls [ bottom of rcvr marked " Converted by Alexander Henry, Edinburgh ", re-blued barrels w/barrel sights, fine scroll engraved rcvr & hammers, etc., case colored hammer safeties , Jones under lever, double trigger extractor, deluxe checkered walnut pistol grip stock & forend, with checkered metal butt plate, appears excellent over all ] Ser # 1708" Gun can be viewed here, http://www.wardsauctions.com/sales/2014/204-guns-2/index.htm
scroll to Index 27, auction item #736A.