Spurious markings on locks and guns are common.
It is extremely doubtful the Joe Manton knew of this, he may have been dead by the time they were so stamped. Which begs the question, were they stamped in Birmingham or in the United States after they came off the boats? I am sure that Joe Manton did not warrant them. But given the communication speed of the time they could have made 20000 of them and Manton would have never known. By "made" I mean that they had a stamp and stamped the plates. The locks were all pretty close to the same quality no matter who's name they bore. Though there were exceptions. But few Americans would pay for a lock that made up nearly the whole price of a cheap rifle like a Leman.
However, he was in debtors prison in the late 1820s. Went broke trying to get the Gov't to pay him for using his patent on artillery ammunition IIRC. Maybe he licensed his name. But why would someone bother when he was in debtors prison?
So the stamping is essentially meaningless unless there is a Joe Manton Hardware store, distributor or importer in the US someplace, which is not likely.
Dan