You can see by looking at them, but I'll describe them for everyone else. There's a semicircular bank of several rows of 25 contacts, with a shaft up the middle carrying contact wipers for each level and a ratchet to turn the shaft. There's a magnet coil on the side frame, left in the picture, with a rocking armature attached to a claw and various springs. The claw rests on the ratchet in the middle. When you apply power to the magnet, the armature pulls in against a powerful spring and the claw clicks over one tooth of the ratchet. When you take power off again, the spring pulls the claw back and turns the ratchet, turning the contact wipers which move to the next contact around on each row.
Resting on the armature is a closed contact. When the armature pulls in, it opens this contact. So if you connect 50V to the coil through the contact, the armature pulls in and breaks the contact, then drops out again because it no longer has power, which closes the contact; and the whole thing repeats, driving the wipers round at about 20 steps a second. In the telephone exchange every subscriber line would have one of these selectors, and when you picked up your phone, your selector would go into this mode and run round its contacts looking for a free exchange line. In the Rotalite, each selector will free run for some random period, probably each controlled by one of the relays. If you poke around in there, press the little rocker at the free end of the relay to close it. 50V won't hurt you. Warning: when the uniselectors run they are VERY LOUD, especially mounted on a wooden board like that. It does no harm but it's startling when it's unexpected, and you can hurt yourself when you jump back in surprise.
The circuit should be very easy to trace. There will be three parts. First, the controls, taking in the relays, selectors and coin parts. Second, the combination logic, consisting of wires running between one row of each selector. Third, the lamp wiring, coming off another row on each selector. The lamp wiring will be entirely separate from the rest, with no connection except through the big blue rectifier.