CASIOTRON 03-502 released by Casio in 1975.
In November 1974, Casio released the world's first digital wristwatch equipped with an auto calendar, the Casiotron QW02, and this model was also released shortly thereafter.
I did a lot of research on the internet, but only learned that Module 02 was released before Module 01, but I couldn't find out the exact release date of this model (Module 03).
Unlike current Casio watches, the serial number on the case back does not reflect the date of manufacture.

It would be a bit of an exaggeration to say that the design, with two buttons on each side of the square case, was a design that had not been seen on analog watches up until then, and that it became one of the standard designs for digital watches that followed.
However, I think this is a very complete design for one of the earliest digital watches.

The SEIKO 06LC was the first in the world to display "seconds" on a digital wristwatch in 1973, but it was not yet common at that time, and this model uses a type of second display in which the gauge above the time display blinks and advances one frame every 10 seconds.
Therefore, at a glance you can tell in 10 second increments, but you won't be able to tell the exact time unless you keep looking at it for a while.
In addition, the far right side of the seconds display is an indicator that shows "PM".
Also, since the "Sun" display is red, it has faded and is almost invisible.

This model I have had a Milanese type belt attached to it from the time I got it.
As far as I can see on the internet, it seems to be different from the type that was originally installed on the 03-502, but I'm still using it as is.
I think the rugged body and delicate strap create a perfect balance, giving it the vintage feel of an old digital watch.What do you think?