(covers information from an alternate timeline)
In an alternate quantum reality visited by Worf in 2370, Starbase 47 was the object of covert surveillance by the Cardassian Union, who had reprogrammed the Argus Array to observe this, as well as Deep Space 5, the Iadara colony and the Utopia Planitia Fleet Yards. (TNG: "Parallels")
Starbase 47 was also originally listed in an okudagram in the episode "Inheritance". The name of the base did not appear in the remastered episode version of the database. According to the commercial transport database graphic in the original episode, in 2328, David Trotti boarded a transport ship at Gamma Towles II bound for Starbase 47.
According to Star Trek: Star Charts (p. 47) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Starbase 47 was located in the Alpha Quadrant.
A different Starbase 47 was featured in the Star Trek: Vanguard series of novels. However, following the events of that series, that station was destroyed and disavowed in Federation records. The designation was covertly reassigned to the starbase referenced in "Parallels". The remastered version of Starbase 6 was later inspired by this version of Starbase 47.
Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385") located this original Starbase 47 in the Alpha Quadrant. The base was operational from 2265 to 2268.