It's from a Killer Instinct 2 machine (dedicated cab) and it's connected between the coin mechanism and the game board, and also to somewhere at the top of the cab where I can't see (and don't want to venture) and probably to the power supply or something. the game board is no longer in use - i've put an ssf2 cps2 combo in there instead and wired it up.
I'm trying to change the machine from freeplay to coins, which is straightforward, but the coin mech will only take £1 before sending a pulse and i want to change it to 20p (I'm in UK).

Seeing as it doesn't appear to be configurable, it's got me wondering if the KI2 game board itself handles the money. I'm gonna go try my xbox KI emu in a sec to see (I'm assuming KI2 will have the same sort of setup), once I figure out how to go to the options...

'coin door interface board'

These pcb's are fitted to pintables too (only with electronic coin mechs).

basically an interface between coin mech and cpu board. All coin inputs run through this board, also voltage to the mech, coin entry lamp.
there's not really anything to adjust on your pcb, some have a data port, some have switches to inhibit different types of coinage.

Re-read your post,

Yes all Williams 90's games have coin/credit settings in the software.

CPS2 only has basic coin/credit settings, 1 coin 1 credit type adjustments. To make CPS2 and the KI2 cabinet coinage work together correctly you would need to add a credit board board, a Klingon, Solo Dispense, Excel. (Suzo/Happ sell these).
The credit board takes all the outputs (£2, £1, 50p, 20p, 10p) from a coin mech, totals up the amounts and provides a single pulse to the gameboard for credit depending on the price of play set on the credit board.