I had a Short Land Pattern musket that had had the markings on the lock largely removed...not recently either. They looks as if they had been scrubbed with sand to obliterate them. I also have a much earlier musket by Humphrey Pickfat that has had the markings defaced although they can be read under magnification.
Legally, being caught in arms against the King with the King's arms was a hanging offense. All British ordnance arms were, in a legal sense, the King's personal property. There are numerous records of Old Bailey trials of person's caught with items bearing "the King's mark" ...i.e. the Broad Arrow, including one of a drunk who scratched a broad arrow on a bag of nuts in the hope it would prevent someone stealing them. Obviously, in the case of the American Revolution this would have resulted in serious reprisals so, wisely, it was never pursued but I can well imagine a semi-literate enlisted man making the effort to erase the official British marks. That it wasn't the royal cypher that was the official mark may well have not been understood then any more than it is today.