Rollers were connected to wheel with a plenty of holes, and there was a led + fotodiode that was used to detect the motion of the wheel. At least that's what I thought back then. (> 20 years, heh)

I also remember just two rollers/wheels (X/Y).

For example, this page: https://computer.howstuffworks.com/mouse2.htm says something about "optical encoding disc".