David Dintenfass kindly sent me an article from the October 1959 issue of Popular Electronics which says, in part, that RCA plans to have 65 titles in the stores in their new cartridges by Christmas 1959. Other manufacturers were shown to be making compatible players.
The article also heralded this as the saviour of tape, and talked about the “old” two-track format running at 7.5 in/s — the cartridges ran at 3.75 in/s (and on some models 1.88 in/s was also available). It goes on to say later that 7.5 in/s quarter track tapes are still a high-fidelity medium. The article referred to cartridges and quarter-track reels as the “one-two punch” against stereo records which seemed to take over from the two-track pre-recorded tapes. The open-reel tape at 7.5 in/s would be the “only choice for the quality-conscious stereophile” since the cartridges were only available in 3.75 in/s versions.
There was no mention of the later name “Sound Tape” in the article, but that appears to be the semi-official if not official name of this format. Thanks to Bill Schuh for that piece of information. Bill also provided a link to The Tape Place which specializes in out-of-print commercial tape releases. I have not used The Tape Place, so this is just being passed on, not a personal recommendation. The URL that Bill provided in 2007 is broken as of 2012, but there is a new link to possibly the same company in a new location.
The Sound Tape cartridges used the standard 1/4-track interleaved format which prevailed for a decade as the consumer open-reel format.
The new The Tape Place has this to say about the history of 1/4-track reel tapes.
7-Inch Stereo Reel To Reel [4-Track] (Music) Tapes were introduced in June 1954 by the RCA Victor Record Company to be used on home Reel To Reel Tape Decks. They have been produced at 2-different tape speeds of 7 & 1/2 inches per second and 3 & 3/4 inches per second. (The faster speed having the better fidelity.) They come in a 7-1/2 inch by 7-1/2 inch heavy outer cardboard box with graphics and song titles on them – that flips open to reveal the plastic reel of tape , which has a paper label with song titles on it. They are threaded onto the Reel To Reel Tape Deck to play them. [Reel To Reel Tapes were discontinued by the major Record Companies by late l975, but Columbia House (Record Club) continued to offer them on select titles into the early l980’s.]