This is a CASIO F-30, the best of CASIO's affordable standard digital watches, the so-called "Cheap CASIO" in Japan.

It seems to have been released in the 1990s, but it does not have an alarm, backlight, stopwatch function, or even a day of week display.
It does not even have a 24-hour display, and AM/PM is determined by the "PM" window in the upper right corner of the LCD.

The CASIO F-30 is a model worthy of the name "Cheapest CASIO," the pinnacle of the Cheap CASIO series, as it eliminates all unnecessary functions.

The CASIO F-30 is simplest not only in terms of functionality.
The watch is also super cheaply made (a compliment). The two buttons are integrated with the back cover, so the back cover is made of urethane rubber instead of hard resin, and the buttons are operated using the elasticity of the rubber. In addition, the urethane belt is also connected to the back cover in an integrated structure.
In other words, all the parts in contact with the wrist are made of urethane rubber, which means that this model is very valuable, without joking, for people with allergies, such as metal allergy.
(The pins that fasten the back cover and the hard plastic of the tailstock also do not come into direct contact with the wrist.)

The back cover and belt of F-30 are one piece, so to change the battery, you have to pull out the pin (not the spring rod, just a rod) on the back side, and then twist the belt to open the back cover, but to our surprise, the back cover is attached to the body with an adhesive (rather strong), which is a terrible design.
The urethane rubber material itself is soft and easily deteriorates over time, causing the belt to tear or the back cover itself to disintegrate when replacing the battery.

The word "disposable" comes to mind, but even so, I managed to replace the battery and the watch is now in operation.

They must have been mass-produced at the time, but for some reason I rarely see them in online auctions.
I wonder if people gave up on replacing batteries and threw them away.
In addition, there is almost no information about F-30 on the Internet.
If you search for "CASIO F-30", you will find only current models of PRO TREK, so if you want to find information about this model, please search for "CASIO F-30 vintage" or "CASIO F-30 Cheap".
(Please do not put a model number similar to a past model on a completely different model by the same manufacturer.)

I also obtained an unused F-30 in its original packaging that was sold at electronics retailers at the time, so I am including a photo of it here.
It is a very rare item, so it is a waste to use it even though it is supposed to be cheap.
(Although it is a stainless steel model, I also recommend the phantom original Cheap CASIO B630.)