Eborac:
I'd guess (really, just a guess) that it's British. You might try these sources to confirm one way or another:
Brown R.R., 1989, Identifying 18th-century trunnion marks on British iron guns: a discussion.
Elvin J.G.D., nd, British Gunfounders 1650-1850.
Ffoulkes C.J., 1937, The Gun Founders of England.
Lavery B., 1987, The Arming and Fitting of English Ships of War 1600-1815.
Don......