I must admit I belong to the school of thought that says it's just for fun so why get all tied up over it? But then I start to think of the fact that it does have to have some rules to prevent people doing the wrong thing. If we have a club series or a National event, there is a percentage of people relatively serious about winning. For example if I decided that next year I am going to attend every HEAVEN meet and win Pre 78 125, then the smart way would be to buy me a 1977 RM125B and go set it up to go fast and handle good. Without rules it's a simple matter for me to add every modern device known to man to improve the power (maybe a later cylinder and head to help that cause), a decent swingarm (maybe a nice alloy one of different dimensions to make the best of the new long travel shocks I bought), and perhaps some N model forks to round out the package. pretty soon I am not racing a 1977 RM125B. Is that really fair for anyone else who might have wanted to win that class (imagining for a moment that I can ride at least as well as Noel and Nathan).
No it isn't.
But having a rule that says I can't modify my bike to have more travel than it came standard with doesn't work in pre 78. If I wanted to race Pre 78 Open, I can get me a Maico with 9" of travel, or even a YZ400D with about the same. But my TT500C in the back shed is a bit light on in that department with just 7.5" at the front, and about 6.5" down the back. Now, a decent aftermarket swingarm and some good shocks and a set of YZ forks and I am away with 9". Or I would have been without that pesky suspension limit rule.
So, it's just not simple, even when you try to make it so. I don't know the answers, beyond a comprehensive coverage of what's OK and what isn't.
I like Firko's suggestion on the other thread of a committee to rule on eligibility. Perhaps this segment of the sport develops its own compendium of eligibility criteria. It can be amended/added to as necessary following meetings by the committee. Over time, the ruless become more and more suited to what actually happens. And grey areas become fewer. And people can see exactly what is eligible and what isn't. In other words, the rules aren't a set and forget formula. The basic criteria are set out, then the lists of accepted mods, bikes, and so on are described in detail. A big job, but it'd help immensely.
By the way, I think there should be no suspension limits for Pre 78, or at least a sensible one. What was the normal maximum in those days, maybe 11"? So, let's just set to 12" and be done with it. If you want to make your stock framed TT500C have 12" of travel, I don't think you'll have to worry too much about whether it even works, you'll be too busy fighting the geometry. And if you are serious about winning, then get a nice Maico or Husky or whatever. I'm not going to lose any sleep over whether or not my 75 RM is competitive and I'll chuckle pretty loudly if I actually managed to beat a 10" travel Pre 78 125!!