Casio says it passed 100 million watches shipped in 1984, nine years and nine months after the first Casiotron. One Hacker News commenter saved paper route money for two or three months to buy a C-80 in fifth grade. Another pointed at the 1989 CGW-50, which showed the planets and Halley's comet, and said it's discontinued and lists in the thousands on resale sites.
If there's an old Casio in your junk drawer, I'd find its model name and look it up on that page before it goes out with the e-waste.
I want the one with the radio.