Quartz 0531 is a digital wristwatch released by Seiko in 1975.

In 1973, Seiko released Japan's first digital LCD wristwatch, the QURTZ V.F.A. 06LC.
At the time, the selling price of the 06LC was 135,000 yen, but this model, the 0531, released just two years later, was priced at 22,000 yen, about 1/6 of the price, so you can imagine that considerable technological innovation had taken place over the past two years.

As you can see in the photo, this model has a super simple design without any of the "buttons" that are almost always found on digital watches, and is a "crown digital wristwatch" that performs all operations using the crown.
Although all operations are performed, there are no functions other than time and date display (alarm, light, stopwatch, etc.)...

Pull the crown to enter time setting mode, turn it towards you to select the item you want to set, and turn it towards you to set the number.
Rather than "turning" the crown, it stops with a click at about 45 degrees toward the front and back.

This watch displays the date but doesn’t show AM/PM, so I was wondering how to tell the time of day when setting the time (though it’s not a problem during normal use). But it turns out that the AM/PM indicator only appears when you’re setting the time. How practical!

What is most distinctive about this model is the way seconds are displayed.
The hexagon in the upper right corner indicates "seconds", which is difficult to express in text.
"The triangle divided into 6 flashes 10 times" → "After flashing 10 times, the light stays on" → "The next triangle flashes clockwise" → "All lights turn off at 00 seconds" Repeat.
Although it is difficult to grasp the time in 1-second increments, it is very intuitive and cool.
I've only seen a similar seconds display method on some G-SHOCKs, but I think it would be good if it became more widespread. I think so.