The polygon mirror is the main part of the printer. It consists of a hexagonal mirror mounted on a BLDC motor aswell as a three phase driver IC. My particular model uses 24V as a supply voltage and 5V logic level, although it's yet untested wether 3.3v work too. When providing power, pulling MOT_ON low and applying a clock signal ranging between 100 - 1000 Hz , the motor starts to spin.
The speed of the motor depends on the clock signal, so far I haven't been able to measure RPM either but I plan to do this in the next couple of days. I'm unsure wether every board is tested individually, but on the back of mine, '24803 min^-1' can be read, most likely standing for the maximum rpm,meaning it's capable of much higher speeds than what is needed for the application.
The pinout seems to be pretty common for polygon mirrors, at least i haven't seen any other so far. There exist some other posts about their pinout, however it's likely that those are wrong as the authors note odd behaviour of the board.
The exact use of the LOCK pin is still unknown to me, however I assume it's supposed to tell the actual rotational speed of the motor. A screenshot of the output in relation to clock input can be seen down below.
Similar to (paper) laser printers, the exact RPM will be determined by counting the delay between interrupts, which will be triggered whenever the laser reflects off the mirror and onto a photodiode. 6 interrupts will then be equal to one full rotation.