RCA Victor Tape Cartridge (1958)

"The RCA Victor tape cartridge (also known as the Magazine Loading Cartridge and SoundTape) was a magnetic tape format designed to offer stereo quarter-inch reel-to-reel tape in a more convenient format for the home market. It was introduced in 1958, following four years of development, at the same time as the stereophonic gramophone record." - Read more...

See one of them in action here.