The black finish is, indeed, ‘black japan’ - a stoved finish common the 19C before other rust-proofing treatments were cheap.
The reason for the square rivets is these are rivets specially designed for lockcases. They are ser either by hand with a punch, or in a flypress. The rivet is first rivetted to the sode of the case, then to the bottom and top plate - so 2 operations. The rivet has 3 ‘heads’, on the side and at each end.
Use of the bullet ward looks continental european, but everything else suggests British. Great quantities of locks and other hardware were experted into America up to late 19C, mostly from Britain but also from Germany.
