A bit of advice:
If you start collecting or repairing mechanical slots, you have to just LOOK inside and EVALUATE how they work. WATCH videos on YouTube showing Mills, Jennings, Watling, etc slot machine in play. Once you understand how they work, then it's MUCH easier to troubleshoot them. You also have to remember not to ever force anything on a mechanical machine or you will end up breaking a part that's difficult or impossible to find and then you have to enroll in a metal fabrication course.
Also that Mills machine is not really an EM or electro-mechanical, other than it may have a solenoid that releases the lever lock, and a light or two inside. The mechanism is entirely mechanical and will work and pay outside the cabinet. EM machines will cycle independently (mechanically) but the outcome of the spin (and the coin-in amount) sets up an electrical action connected (through bundles of wiring) that puts the hopper in play in order to pay out much larger amounts of coins as compared to a truly mechanical machine.
That particular machine is known as a Mills "M" slot.
Also, check out coinopcollectorforum.com - That forum is exclusively for mechanical slot machines.