You shouldn't have to open the PSU case to adjust voltages.
Open the front PCB access door. Look at the PSU there are two small holes at the back of the PSU (when viewed from inside the cabinet).
One hole is for 5V adjustment and the other is 3.3V Adjustment (When running JVS power with a 001 loom for example)
5V and 12V are shared voltages so adjusting 5V adjusts your 12V in accordance.
The screw post on the back is the Ground / FG for the cabinet/PSU. Its on a screw post because the intention was to daisy chain a ground wire to every cabinet in the game center in Japan. Not all outlets in Japan had a dedicated ground pin yet so this is the reason for that being there.
If you run a grounded IEC power cable you can ignore worrying about that grounding post.