The setting knobs and the back side reversing knob used to be round in the early Curta Type I models. Does anybody know at (about) what serial number their shape was changed to the common finger-friendly design with bigger grip? The highest serial number with round knobs that I have seen in a photo was 7089. The lowest serial number with the new knobs that I have recently seen was 8394. Thank you for an answer.

Also, I believe to have seen different designs for round knobs. The earlier knobs seemed bigger and had a better grip while the later round knobs were small and smooth. Is this correct? Why did they change to a worse size? (I have pictures but I do not know if and how I can attach them to this post.)

I'm also curious about the early Type I design evolution.

Curta s/n 7676 has rectangular sliders and the older right-hand thread case style. There was a Type I on eBay (s/n 4483) with large textured round sliders. I've also seen photos of earlier Type I's with the smaller round sliders (s/n 2805, 4336).

Do you mean these two different designs of knobs?
The bigger ones are only plastic caps for a better grip on the small standard round knobs.

The Type I setting knobs changed from "pin" style to "rectangular" style in this manner:

setting slider knob - pin knob: serial numbers 1 - 7178 and 9321 - 9620
setting slider knob - narrow rectangular knob: serial numbers 7179 - 9320 and 9621 - 68180
setting slider knob - wider rectangular knob: serial numbers 68181 - end

The pin knob was probably used again for awhile to use up parts that were in stock.

It is unclear when the plastic "mushroom" caps became available as an add-on option for the pin knobs. A mushroom cap was generally not applied to the reversing slider because the outer diameter of the cap prevented the reversing slider from moving all the way up to the bottom edge of the "upper body collar".