Sounds like a Field's patent rifle. H&H did build a few and some of the H&H serial numbers are listed in Kirton's British falling block book on p.207.
Do you know the serial nuber?
Looks like many were chambered in .303 but the larger bore rifles look to be in BP express chamberings. I would think that when loaded within reason, it would surely handle 45/70. Is it not barreled now or is it "shot out"?