If the OP found this lock in East Anglia (he says a local market), then German/Low countries / France as origin need not be so surprising.
The Hanseatic League flourished 13-17C, started by German merchants. Although it is particularly associated with Baltic trade, league traders also traded with Britain, having an important base in London. In those centuries, before the river silted up and ships became larger, Norwich was a major east coast port, and received much trade from the League. That included, inter alia, ironmongery. Indeed, before our canals were built, sourcing some things in Britain, particularly along the east coast from Essex to Hull, was easier from the continent by ship than from within Britain!
https://upload.wikimedia.org/wikipedia/commons/8/86/Ausbreitung_der_Hanse_um_das_Jahr_1400-Droysens_28.jpg
This explains the presence of some continental stock locks, especially in Norfolk (and elsewhere in East Anglia particularly) churches, where they still survive in service.
Later, refugee Huguenot metalworkers from France and Low Countries settled in East Anglia, particularly Suffolk, and made ‘strong German coffers / Flemish kistes’ (aka ‘Armada Chests’).
Some other Huguenot refugees later made it to Willenhall, notably Charles Aubin, and Mr Duce. (I cannot find his Christian name.)
