Further correction (sorry):
NO rifles were ever marked "1878", though it is an easy mistake because some really do look as if they are. However, in ALL cases, the last digit is a too-deep "3". It is not clear whether this was due to a bad die, too much pressure, block slightly "out of gauge" (too thick) or what. Also, it is entirely proper to see the 1873 date well into 1887, as older blocks - which were identical save for the date - were used until supply was exhausted. The "1884" blocks appear well after the adoption of the model change, which basically referred to the new "Buffington" rear sight, as nothing else was changed at that time.