Yesteryear's home music server : twenty cassette sides, or ten hours or a full day's work of music.
Of course, indexing what is on each tape mostly relies on memory, human memory, which makes for a very good exercise.
Up to ten cassettes can be stored in a JL-10 magazine and each can be played on both sides thanks to an auto-reverse head : you can see how the system works here or here
Each JL-10 can store tape type (II/III/IV or I) and NR type (off / B / C) for each cassette inserted with two switches. Push the JL-10 inside the MTL-10 (motor activated !), close the door, it works.
Of the colors made available, only the almost-black and silver are somewhat reachable ; the blue and red are alas extra-extra-rare. The JL-10 was also theoretically available in three colors but black is the color we all got.
(I am the lucky owner of a NOS/NIB silver MTL-10 with three JL-10s :)