The LB relay fires for the last ball, and the 4B relay fires for the A-B-C sequence completed. Either one by itself will give you a 2x bonus. If you look at the two schematics in reply #40 either one of those relays firing will change the paths used to count the bonus from the 1st (1x bonus) schematic to the 2nd (2x bonus) schematic. So last ball or sequence completed (but not both) will use the paths highlighted in the 2nd schematic.
@paulace sent me a video of a miscounted 15,000 point 2x bonus offline. It took playing it over and over at half speed but I think what I heard is this:
As described earlier the Score Motor 4A an 2B switches generate 6 pulses each time the motor turns through one cycle. The 6 pulses all find their way to the 1000 point relay to add points. But only every other pulse, specifically the even pulses, should get to the Bonus Unit Subtract solenoid. The net effect should be 2000 points for each Bonus Unit step.
I think what's happening is that in each Score Motor cycle one extra pulse, pulse number 3 of 6, is somehow getting to the Subtract solenoid. So for each complete score motor cycle 6000 points are scored but the Bonus Unit steps down 4 times instead of 3 which results in a 1.5x bonus instead of a 2x bonus. This will vary a little based on how many steps the Bonus Unit takes during the last Score Motor cycle because once the Bonus Unit reaches zero, the E relay relaxes and no more stepping or scoring happens.
So the question is how is the #3 pulse getting from the Score Motor 4A switch that generates it to the Subtract solenoid? That's not obvious:
The 3rd Score Motor 4A pulse doesn't really overlap with any of the other switch stacks. So there's likely a maladjusted switch or a short somewhere. It's not a dead short because pulse #1 for example doesn't get through but it's a repeatable short.
You might try unplugging the game and slowly turning the Score Motor by hand to the point where the the 4A switches close for the third time in a cycle. Then have a good look around to see what other switches are closed. Then see which of those switches is open the first time the 4A switches close.
If this doesn't identify a culprit you could (if you're comfortable working with the power on) queue up a bonus, unplug the Score Motor service jack, close the outhole switch and slowly turn the score motor by hand until the 4A switches close the third time. The Subtract solenoid will probably make a nasty noise but use a piece of paper or something non conducting to open up the closed Score Motor switches one by one. That might reveal the problem.