May I just add to the general understanding that an FIA logo on an event only applies to the International Calendar registered race and this from period.
For example, when WSC Championship visited an event back in the 60s, 70s or 80s and up to now, only the FIA Championship races were FIA sanctioned, hence that the Spa 6 Hours for example run the Masters FIA Championship let's say, so only these races are subject to full FIA standard and strict enforcement of the HTP Papers. And same in period when WSC would run on the Nurburgring, the DRM, running to German rules was not concerned.
Adding another statement is important, HTPs are today mainly enforced by series or promoter as they say they are "sort of" running to Appendix K ruling but in situ, it's pretty much different.
While people, do or do not agree with the system, I was 9 or 10 by the time the proper and so-called FIA trophy for GT & TC falled appart in the early 2000s, a sad loss which happened because of the arrival of a promoter only looking at numbers...
However, what I remember is that cars were before that properly scrutineered back then, some of you might remember how terrible it was to face the scrutineers at Nürburgring... There was far less cheating/funny cars because the scrutineering was good and rules were back then, meant to be enforced. Not just promoters and organizers saying, "we're appendix k" but allowing stuff to sort of make the number and please customers as it is today.
I believe that to stand strong, it is a matter of taking the risk to enforce something, it might not be good in terms of numbers and finance at the beginning, of course, but in the end, stand strong and once people understand it, it works out better because there is a common line. We've ended in a world of historic racing which is more of "customer historic racing".
I walk the HTP line every day and I'm just unhappy to see historic cars ending up with funny aeros, or funny engine and drivetrain or being built by people not willing to understand history but only willing to sell and/or race performance and seeing the "good old boys" walking out of the sport because of some practice and cars being "funny".
HTP is not the sole problem, performance and how cars are now today handled, build and developped is a far bigger problem and maybe it's the reason why HTPs are getting more and more complicated to get... Just saying...
By the way, national/club papers exist and are eligible for some races within the issuing country but as we've witnessed, when someone builts a d-type with carbon fibre bodywork or full electronic injection and single throttle body on a Group 44 XJS Trans-Am car, is this really the spirit and what we want as historic racing ? I am happy for them to stay national/club cars, it's their fun and toys after all but they should race where this is allowed.