T.Altham and Son of Penrith were Ironmongers and Iron founders .There business was established in 1831 and survived till 1981.
What you have is almost certainly a Birmingham made gun engraved with the Altham name,sold through there Ironmongers store[Hardware store].
Many ironmonger/hardware stores located in the U.K sold Birmingham and in some instances Belgium made guns engraved with their name and address.

Roy Hebbes