They were primarily intended for use in automobiles and were an enormous improvement over the expensive turntables available for cars that would skip on smooth roads, much less bumpy ones. Those played the bottom side of a 45.
The tape was a continuous loop that came off the inside and then wrapped around the outside. Hard on the tape but an ingenious design courtesy of Bill Lear.
The original Learjet tape housings were assembled with screws and could be internally cleaned. Later cartridges were hard to open without destroying them. Head azimuth was a real problem, very few playback decks had adjustable azimuth to minimize crosstalk and improve treble response. The matchbook system mentioned above was much more common.
They had a pretty short heyday, like many others I switched to a Sony cassette player in 1970 or ’71.
Lear was an incredible guy, struck out repeatedly but hit some homers, too. Motorola and Learjet did pretty well. Motorola made the first car radios; people said they would distract drivers and cause wrecks. The 8 track was a segue from his car radio concept of the 1930s. When he died he was working on a steam automobile project.