According to "The Remington-Lee Rifle," the Army bought a number of Lee rifles in the 1884-85 time frame and by late 1885 750 had been delivered and over 700 of those issued for troop trials. In annual reports between 1886 and 1891 there are mentions of Buffington sights on some of those rifles--possibly as unit replacements. A Boston firm bought several hundred of the Remington Lees from the Army in 1891, and the Navy promptly inquired about buying them for the Massachusetts Naval Brigade. Apparently a deal was struck and in 1892 208 of the former Army rifles were issued to the Naval Brigade. The Brigade marked its rifles the way yours is marked, with letter, over numeral, over "NB"--some were stamped and some were painted. Both the barrel and receiver should have "DFC" stampings for the Army's inspector, and there may also be a vestige of the original DFC Army cartouche in the stock, although the reports about the Massachusetts surplus dealer suggest some degree of refurbishing that could have affected the markings.
Sounds like a rifle with an interesting history--not very many of them took that journey.
--DJ