I wash them under sink faucet with tooth brush and dish soap. Fling off and blow out the majority of the water. Then sit in front of a box fan for a couple hours. Unless the board is burned they usually come out looking sharp compared to before washing. The amount of smoke/dust/crud that accumulates on many boards over 30 years is amazing and pretty gross.

Not all system 7 boards don't have dip switches but I think some early ones do. I will honestly wash boards with dip switches. I avoid drenching them but also make sure to blow out the water best as possible. A wet dip switch can for sure cause problems tho. In a Bally game wet dip switches will cause switch errors. Are the LED digits sealed? I don't know but probably are. The plastic box flipper relay on the driver board is not sealed and water will get trapped inside of it.

Water soluble flux is awesome. I use Kester 24-6337-6403 when assembling new PCBs. Rinse under warm water with a little toothbrush action and all the flux comes off easily. Dang rosin flux is a pain to clean. No clean flux looks like hell to leave on a PCB and just as hard to remove as rosin. I will even use water soluble flux when doing rework where I know im going to wash the board later. Under PF stuff, rosin.