British libraries - I don't know how it worked overseas - registered each recording with the MCPS and the PRS soon after it had been given a title, in order to receive the mechanical and/or performance royalties it may generate once it had gone out to editors, TV and radio stations etc. So there was always an interim period when a recording was on the MCPS's and PRS's books but had yet to appear on LP, 45 or, if we're going way back, 78. Such titles were registered under 'T' numbers, indicating that for the present they existed purely on tape.
KPM, and subsequently Bruton, kept what they called a "book of 'T' numbers." So, for instance, if a client visited Bruton's office, and there was music from a recent session which had yet to appear on LP, but which Robin Phillips and Aaron Harry thought might interest the client, they'd play the client the tape and explain that the music could be licensed even though it was still only on tape. And thus that music's usage would be processed through the MCPS/PRS under its 'T' number(s).
Some of Francis Monkman's 'Energism' material was licensed in this manner before being issued on LP. John Scott's 'Thriller' music on BRJ 4 'Thriller/Suite of the Hurricane/Suite of the Shark' (1978) was certainly initially registered under 'T' numbers because, as many of you probably know, that music was originally written for the 'Return of the Saint' TV series via a deal with Bruton/ATV Music. In return for paying for the sessions, Bruton was allowed to issue the music following its usage in the series.
