From earlier in the thread:

The score reels are read optically - I briefly considered just counting the AC pulses sent to the reels, but we all know these reels don't necessarily always show the number of pulses sent to them, and it was more important to me that the lcd show the actual score according to the reels, not what the score *should* be.

Using character recognition - obviously, too costly to have to include 4 cameras/optics etc.

Reading the edge of the reel - His test bed is a Williams EM, their score reels have an appendage on the edge of the reel as part of the ratchet. He could monitor that but then, there is no indexing to tell him what is zero and what are the other 9 numbers.

He is using optical couplers to monitor the 10 binary outputs of the Wiper PCB on each reel. Problem here, there usually is not one on the 100's reel. And, that's a lot of inputs to multiplex (which was touched on when the conversation turned to multiplayer support).

Or, like you mentioned, he has added additional optical encoding (black marks vs white reflective plastic) and decoding long and short lengths as they pass?

Otherwise, he is doing something I haven't envisioned.