(NA; full name National Association of Professional Base Ball Players).
There were two such teams, a very short-lived one in 1875 and another which (in the opinion of some sources) was a precursor to the modern St. Louis Cardinals.
Because the St. Louis Brown Stockings continued as a charter member of the National League and completed two seasons there (1876-1877), theirs is the more important place in baseball history.
Probably for that reason, the Brown Stockings usually get "STL", nearly standard as a three-letter abbreviation for "St. Louis" in baseball encyclopedias, where space is severely at a premium.
For example, Pete Palmer uses "STL" for the Brown Stockings and "RS" for the Red Stockings in print (see Total Baseball or the new Baseball Encyclopedia); Baseball-Reference uses "STL" and "SLR" online.