Too big for a Bess lock, it must have been for a wall gun, or punt gun. Hungry Horse
A similar lock was posted on the British Military Forum in March. That post and the subsequent responses describe their usage on cheaply made trade guns used in the slave trade that were marked with the Elephant and Castle marking of the "Company of Merchants Trading to Africa". It is very interesting reading!
The fall 1966 issue of The Bulletin of the American Society of Arms Collectors illustrates this mark in an article by Charles Hanson, identifiesit as, "British African Mark" , topic of article is trade guns.