have you double checked the gapping on all 500 point leaf switches, wonder if you just getting a hit from vibration. Also, when it switches from right to left there is relay that handles that shifting (on my game it has a little buzz when active to one side (right I think), I don't see you making any comment about that relay (I don't have the schematic in front of me), but I would also look at what is passing through that or near that (e.g. relative to a gap issue).

I certainly may be wrong, but since the 1,000 point relay is pulsing 5 times, this 500 points may not be coming through the 500 point relay, but as a result of the 5 pulse to the 1,000 point relay triggering through to the 100 point relay via some path independent of the 500 point relay.

I certainly may be wrong on this, but the spinners go through the right and left bonus units, just like the top eject does, so if a spinner is stuck closed when in resting position, I believe that would result in the 500 points as well.

Williams_1976_Grand_Prix_Manual.pdf (page 33 of 36).png

You should also be checking the switch on the right spinner relay vs the spinner itself. I have found that if the spinner itself is an 'closed' state other aspects of the game will not behave correctly.

If that is activating, then it is probably not the spinner, since that is a 100 point pulse. Certainly makes sense though to take a closer look at the 'change' relay, since a bunch of things run through that.

How are you advancing the match coil to switch from left to right for your testing?

I wonder if those are considered the 'stand-up' targets that feed the 500 point relay as Chrisbee pointed out, if so, I would check carefully there for a stuck switch, etc. hmm, I may have to go take a look at that tonight.