The eprom says "Mikohn" so the board is maybe for a random jackpot that gets awarded the longer you play. I think it was meant to help keep players at the machine longer.
Here's part of an old write-up:
Mikohn Gaming started their corporate life as Mikohn Inc. which in 1986 they implemented the know how required for progressive slot machine jackpot systems and it is the technology which is found in every land based casino to this day, who knows if it wasn't for them progressive slot machines may never have been born!
They also developed the often found Mystery Jackpot software which can be found on many modern slot machine and lets players, win, totally at random one or more of the ever increasing jackpot payouts. while Bonus Jackpot software allowed for random jackpots in addition to the regular payout.