Eeypiece means the lens assembly for viewing the groundglass-/aerial-image.
Diopter means the refracting power of that assembly.
There also is a military eypiece at guns that is called diopter. I guess that is causing confusion.
But keep in mind I'm only a technical minded non-native speaker...
Sorry the Leica II had the same adjustment for the rangefinder but it was around the outer of the eye piece for the rangefinder.
Think the Leica II was about 1930-31, try Google for photo I've only got a nicca which is a III clone, with same optics.
At least in terms of the Bolex reflex models, the image is quite dim, so a tight fit for the eye is needed. Making eyeglass wearing not practical. Many, many still cameras work fine with eyeglasses. Therefore not as much need for a corrected eyepiece.
Some exceptions in my work:
Rollei 3003/2000F cameras work better with the eye close to the finder; like a movie camera. Therefore have an adjustable eyepiece.
Bessa R4 (21mm finder) only useful for 21mm if the eye is close to the finder. It is not adjustable, but takes common Nikkor diopters.
I'm sure there are many other still cameras that function poorly with eyeglasses.
Typically in my experience with many models, the eyefinder lacks viewing angle when used with glasses. On the other hand, if one is presbyope one still needs the glasses to read/manipulate controls at the outer of the camera.
In addition to the optical adjustments, mechanical adjustment to compensate for parallax in RFs would be needed.
One early example is the viewfinder of the Voigtländer Vitessa where the ocular could be slid some mm. diagonally from the infinity- to the 3,3ft. setting. In the Leica M the movement of the frame was coupled to the rangefinder cam.
In addition to the optical adjustments, mechanical adjustment to compensate for parallax in RFs would be needed.
One early example is the viewfinder of the Voigtländer Vitessa where the ocular could be slid some mm. diagonally from the infinity- to the 3,3ft. setting. In the Leica M the movement of the frame was coupled to the rangefinder cam.