Notice the inset of one of the wheel feet. It corresponds to one of the two black cylindars, but it is flipped according to its position on the mouse. What you can't distinguish in this view are the vertical grooves along each cylindar's side. The cylindars sit at different tilts and so do the roller feet. This must be kin to the Honeywell mouse.
These cylindars are like the encoder wheels on most ball mice which signal an LED light source to the photodetectors as the wheels turn. Turning the wheels interrupts the light beams which causes pulses. The software interprets these to situate the cursor.
You can also see the three button switches, little beige bars inside black blocks.