Paul, when you visited the USMC Museum, did you see any of the Springfield Armory-made Remington Model 1870 rolling block rifles or carbines in 50-70?  They were marked US Navy (or maybe just USN) with an eagle on the right side.  They were in use by the USMC throughout the 1870s.

On another forum, a guy working in Korea found some fired cartridge cases at a construction site.  Turned out, the USMC was in Korea about 1871 and did some shooting.  Stuff most of us never knew.