Just a hunch: They may have been built at the end of the BP era with BP chamberings and proofing out of Damascus; and shortly after have had fluid steel, nitro proof barrels retro-fitted to keep them in service as BP became obsolete?

There is something a bit special about the AH hammerless underlever. Mine retains its original Damascus barrels and 500 No2 Exp chambering, built in 1891; it has a patent use number fewer than 100 higher than your .500, and was built in 1891.

Has anyone yet written a definitive book on the maker and his wares?