I think most EMs don’t have a board for the 100s reel. There is one on the 10s for the match function. There is one on the 1000s and 10,000s, those two are used for the high score award function.

Might be the motor switch that actually scores the 500 or 300 points is just undergapped, overgapped or dirty, but don't know if both of those would go thru the same switch. Are you getting the correct number of chime rings or is it also missing one just like the score?

The issue has nothing to do with having a circuit board on the reel. Make sure the reel isn't binding, then check the motor switches which pulse the 100 point relay. Check the lever which moves those motor switches. I had a game once which sometimes gave 4 or 40 points instead of 5 and 50. Seems like someone tried to turn the motor backward, and bent the end of the lever which pulses the switches. Straightened it out, and all was good.
Check the 100 point relay as well. Tighten the switch stack screws, then clean and gap the switches. By any chance, is this a reimport game with a 50 HZ motor?

I agree. Check your motor switches. Sounds like one may be dirty or may need to gap adjusted. (it seem to work 4 out of 5 time). also spot check the motor "teeth" and make sure they are all there and one is not bent.

OK so I had this same issue on my first machine and subsequent machines thereafter.. usually happens with the 100s but can also happen with any other real as well. If your scoring is missing steps and such it is usually an indication that the plate on the relay for that score reel has been magnetized. you can verify this by sliding a thin piece of paper in between the relay plate and the relay itself. If the problem goes away then you have 2 options, disassemble the relay and demagnetize the plate ( you can search this forum on how to do that, or buy a new one ) or place a piece of electrical tape between the two parts as a semi permanent solution. Number 2 is what I do as it works fine, is effective and does not require a disassembly of the relay.

Another thing to look at is the finger spring that engages into the reel and keeps it from reversing. I've had reels that didn't score correctly and it was because that spring wasn't engaging correctly and when the reel went to step it would reverse slightly and usually come up one step short.

If the chimes aren't correct though I'd check the 100 point relay to make sure all the switches are right. The bane of EMs is switches that looking like they are making but aren't, and vice versa.