The patentee of #324 of 1870 was John Thomas, a Birmingham action maker. Apparently he never made complete guns, but supplied his barreled actions "in the white" to many other gunmakers. His patented single-bite snap action as seen on this gun, had a "turret", turned by the toplever, inside the breech. This turret bears down on the bite, projecting from the rear of the barrel lump.
John Stanton of Wolverhampton was a specialized gunlock maker, hence the "GL"? Stanton invented the rebounding hammer lock in 1867 and supplied his famous locks to "name" gunmakers. My Rigby .500 BPE DR has locks signed "Stanton". He also supplied his locks to Purdeys and Holland&Holland.
The gun was proofed in London, London View marks on the action flats. This gun may well have started life as a second grade Purdey, made to order on a Thomas action with Stanton's "new" rebounding locks. The sorry remains of engraving point to Purdey's direction. In the 1870s Purdeys some times used actions by other makers, fi in the Coburg armory in Germany there is a Purdey signed double rifle on a Westley Richards action. It once belonged to Ernst von Sachsen-Coburg-Gotha, brother-in-law to Queen Victoria.