Hmm, All along time ago now.
Don't over think it, there may be many combinations for all the fruit symbols etc but the MPU isn't interested in that. All there will be is a handful of inputs to the MPU.
A couple of switches is all you probably need, one for each input. You probably don't even need to look at the reel deck.
There may be 10 different combinations on the reels to pay 20p for example but the MPU will only need one input. (one switch will cover all 20p combinations).
On the Electro decks with the optos they count the positions and increment the RAM, the MPU then