Countermarked 1816 Halfcrown

very interesting: the "Mouse" is very neat - seems well-aligned, although not evenly spaced.
I'd think it likely that the 1881 was put there by a different hand, at a different time, although, of course, I can't say which was earlier.

Is "Mouse" a personal or business name in Britain ? ... can't say that I've ever come across it.

Some see these as vandalied, or disfugured: I prefer to think of them as "Coins with a story".