CITIZEN CALCULATOR was released by Citizen in 1976.
Calculator function buttons are arranged around the dial, so you can tell at a glance that this model has a calculator function.
Citizen released its first LCD digital watch in 1974. Liquid Crystal Therefore, it can be seen that technological innovation has progressed at a tremendous speed in just two years.

Old Citizens are characterized by the model number etc. being very difficult to understand, but in the case of this model,
　・Model number (module): 9090
　・Product control number (catalog): LC-909
　・Model number: 59-2021
　・Side number (written on the case back): 4-095928
　・Serial number (serial): 6120xxxx
It seems like.
The first three digits of the serial number represent the last digit of the Western calendar + month.(Manufactured in December 1976)
Also, the model number (module number) is not written on the back cover, but only on the internal module itself.
There is also a product control number, which is similar to the model number but is listed in the catalog.This is also not written on the back cover.
It's so confusing that I don't understand what's going on...
For models newly released after September 1981, the above model number, model number, and side number are unified, and the first four digits of the side number represent the model number.

Also, the "GN-4W-U" written on the case back is not a model number, but a "structural symbol" on the exterior, indicating how the windshield/bezel and the body are fixed.
For this model starting with GN-4W, the windshield is press-fitted into the bezel, and the bezel is press-fitted into the body.
More information about structural symbols another time...

Pressing the "ON" button at 10 o'clock turns on the lower LCD (calculator screen).
While in calculator mode, the "ON" button remains pressed in, and pressing the "OFF" button returns it to its original state.
Also, at the top (12 o'clock direction) are the "Light", "ADJUST(S)", and "Date display switching" buttons.
By the way, the light (small bulb) is attached to the side of the time display, so it is impossible to calculate in a dark place.

There is also a model with a similar design to this model with 28 buttons (equipped with memory functions such as M+/M- buttons) and a model with 41 buttons (equipped with scientific calculator functions such as trigonometric functions and logarithms).
The size of the body itself is the same for both the 21-button model and the 41-button model, so the 21-button model is the limit for pressing buttons with your fingertips, and the 41-button model is probably impossible.(I haven't tried it since I don't have it)
Therefore, you actually need to press the button with something like the tip of a toothpick, and the top of each button is slightly concave to keep this in mind.

The other day, I got a model with 28 buttons, but unfortunately it didn't work.
It can't be helped because it was made over 40 years ago and is a very special digital watch.