Wally...Your XT500 slider fits into pre '78 and the slider class ( 18.10.0.1) with the Dec 31 1976 cutoff which somebody has adapted from the old 2 valve class. It will never however be allowed in the pre '75 division which I believe was your original question.
To me this ALMOST lets in XT powered sliders [pre dec 76].
No ALMOST about it Wal, as far as I can see it fits OK into the slider class.
This is just another example of the eligibility criteria needing a good overhaul. With the new era cutoffs coming into the sport, each with its own quirks and foibles, if the rulebook isn't tidied up and made more user friendly eligibility security is going to be a nightmare in the future. The grey areas have to be made clear so that situations like Wals or more relevently what happened at the Conondale Nats can be avoided in the future.
Thankfully, this long overlooked overhaul may soon get underway if recent moves come to fruitition.