This is the Q&Q AUTO CALENDAR, a digital wristwatch released by Citizen in the 1980s.
I did a lot of research before posting this, but there was no information about this model on the internet.

As I mentioned before when discussing another Q&Q model, Q&Q is one of Citizen’s sub-brands, and stands for “Quality & Quantity” (bringing quality products to more people).
Most of the products are basic models in the low price range, and are in a similar position to Cheap Casio.
Therefore, a considerable number of them must have been produced at the time, but more than 40 years have passed since their release, and there are extremely few left in working order, so it is thought that there is no information such as photographs.
Furthermore, while numerous “CITIZEN” catalogs can still be found online today, it is believed that no catalogs for the “Q&Q” brand were produced at the time—or at least none were distributed to the general public—and no printed materials from that era remain.

The operation is extremely simple: press the red button to enter the time setting mode (seconds will flash), press the black button to advance the numbers (reset in the case of seconds), and press the red button to advance the blinking position as seconds → minutes → hours → month → day.
Although it cannot display 24 hours, the A/P display will appear properly when setting the hour.
When the time is normally displayed, the black button (only while pressed) displays the date.
Personally, I think this method of operation is the most intuitive and easy to understand.

After all, the biggest feature of this model is that it is "ultra-thin".
The actual thickness is approximately 4.3mm (excluding the button part), which is quite thin even by today's standards.
The thinner the watch body, the cheaper it tends to look, but this model's design has a black body with a gold frame, and while it doesn't feel luxurious, it doesn't give a strangely cheap impression.
When you open the back cover to replace the battery, you will see that there is also a hole in the circuit board where the crystal resonator is attached, and you can see that they were trying to somehow fit the module inside the thin body.

In addition, the belt attachment part is made with a special method that allows you to insert the belt from the side, probably because it would be thick if you use the regular spring bar type.
There are models in CASIO's ultra-thin PELA series that use a similar type of belt system.
The belt in the photo still retains its flexibility and is in miraculously good condition, but even back then, if the special belt broke, it could not be replaced, so it seems that the whole body was thrown away, and I think that the special belt method is one of the reasons why there are so few copies of this model in existence.

In addition, this model cannot set the "year", so I tried various things to find out what "AUTO CALENDAR" means, and it seems that the name "AUTO CALENDAR" means that the date only needs to be readjusted once a year (March 1st).
By the way, February always counts up to 29 days, so leap years can be counted as is.

If you think about it, this model does not have an alarm, light, or stopwatch, so this (auto calendar) is the only function that can be written on the watch itself.
Come to think of it, if you look closely at the CASIO F-30—the cheapest Casio model, which, like this one, has no functions other than time and date display—you’ll see that the paper tag says “Auto Calendar.”
The F-30 has "Lithium" written on the watch itself, but this simply means that it uses a lithium battery (CR battery), and has nothing to do with the function of the watch itself.