It's the Muntz Stereo-Pak,used for playing stereo music in car tape players.It's later spin-off is the 8 Track cartridge,also used mainly for playing stereo music in car tape players.The 8 track offered twice the amount of music in the same size cartridge and was of simplified construction.
You're close. The Muntz is also a spin off. The pictured cartridge was developed in 1954,
for a specific medium. If you "clap for the Wolfman" he'll take you too it.