This is a beautifully designed and executed mechanical calculator, of which I own a model II, serial number 532235, bought in April 1968.
You can find many pictures at curta.org. I have a booklet and a quick manual.
One of the little known facts about it is that even the little “commas”, or “thousands” indicators, are all metal. They are very tiny. They slide around in a groove and are used to remember where the decimal point is. You can spot them as little whitish dots around the base and the top of the machine.
If you remove the lock screw, you can carefully slide one out (watch out! they are spring-loaded, so they jump far away and become unfindable if you don’t hold your hand over the exit point when removing them!). You can then see that they consist of three parts: the body, a spring and a little hard ball. It is this combination that provides both the friction and the ease with which they can be moved.
Below you see a drawing and some photos of these commas:
the lock screw which allows you to slide them out.
the three parts compared to a grain of rice.
Close-up, showing dimensions.
(photos made with ProScope)
The instruction booklet: