The CASIO F-100 was the world's first wristwatch with a hard plastic case, launched in 1978.

CASIO, a latecomer to the watchmaking industry, succeeded in reducing the cost of digital wristwatches by adopting a hard plastic case for the F-100, and this success is considered to have been the basis for the success of subsequent G-SHOCK and other models.

The selling price of the F-100 in 1978 was 7,500 yen, less than half the price (15,500 yen) of a stainless steel body model (52QS-14) that used the exact same module (Module 52).
However, the F-100 was adopted as the wristwatch of Ellen Ripley, the main character in the movie "Alien" (1979) (the original model consisting of two F-100s connected vertically), and has long been a particularly popular model among older digital watches, especially in US.
The F-100 fetches much higher prices than the 52QS-14 at online auctions.

All four of the F-100's control buttons are located on the front of the body, and each button has its function indicated by a "color" next to it.
White (upper left) is "ADJUST," yellow (upper right) is "LIGHT," red (lower left) is "MODE," and green (lower right) is the "DATE, START/STOP" button.
It is both intuitive and unintuitive that the colors represent the functions...
I think it was a desperate measure because the buttons are located far from the windshield (on which letters can be printed), but perhaps it was the fashion at the time, since CITIZEN's DIGIANA also has colored buttons.
Also, having the buttons on the front is a reasonable design for molding plastic (it can be die-cut in one shot)
If the F-100 had sold more explosively, it might have become the norm for plastic-body digital wristwatches to have buttons on the front!

Also, although black wristwatches are quite common today, before the F-100, wristwatches with black bodies were basically metal or gold-plated (with the exception of a few such as CITIZEN's "Blackie"), so black wristwatches were quite rare.
Therefore, in Japan, the catchphrase of the F-100 in pamphlets was "Black Quartz（黒のクォーツ）".
By the way, "Black Quartz" is pronounced "KURO-NO quartz" in Japanese, which is similar to "CHRONO quartz".

To switch between time display mode and stopwatch mode on the F-100, press and hold the lower left button (red).
There are many unique ways to operate old digital wristwatches, and if I don't write them down somewhere, I'm likely to forget them.