Hello bidault,
The reset button is in the upper right hand inside corner of the cabinet, right above the white cherry switch. It's yellow and should have a label below it..."RESET SW". The "RESET SW" will reset the codes as labeled on the inside of the door. Error code 12 is a battery polarity issue. There could be a few reasons for the error.
1) The NiCad battery voltage is low. The error will occur if the machine has been left off for a period of time and the battery has discharged to a low voltage level. There's a charge circuit on the CPU board that maintains the battery voltage. Leaving the machine powered up should restore the voltage level and that's providing the battery is good and the charge circuit is functioning properly.
2) The NiCad battery voltage is low and will not accept a charge. The battery must be replaced with a rechargeable one.
3) The NiCad battery voltage is low due to the fact it is old and has started leaking acid on the CPU board. The battery acid has damaged the CPU board circuit traces and in some cases the components mounted to the board. A board repair is necessary.
When the above conditions are corrected the yellow reset switch will clear the fault.
There is another master reset button. It is located at the bottom center of the CPU board (mounted to the left wall of the cabinet). You may have to remove the coin hopper to gain access. The button must be held down for about 5 seconds while powering up the machine. This will reset-clear the board memory back to default settings.
I noticed in one of your photos that there appears to be some cover corrosion at the CPU board reset button hole. That's an indication the battery has failed and sprayed acid on the cover. Before you power the machine back up again I would unplug the wires to the board, remove the handle retaining screw, pull out the board, remove the cover and inspect the battery and it's surroundings. If there's a battery issue we need to address it before the board is wiped out entirely.
Keep us updated.