The HP427 I'd have to guess is high performance 427. As for the exact year they came out I'm not sure. Mickey Thompson has been around FE parts since the the very early 60's. Those are hard to find and they are one of the few parts unauthorised to use the Ford script and oval. And get away with it. Plus they have not been butchered with vents or pcv which most are. You can get the chrome "power by ford" VC's on ebay as repops for $160. Original sets are a little more with nice chrome. The chrome GT dressup package included the air cleaner lid, VC's and the oil dipstick. 67 did not use the snorkle just a round aircleaner with mesh vents on the side. So if your car came with one hang onto it. Even if you don't use it they only go up in value. I should mention the std 390 didn't have the GT dressup package and just used painted parts where the chome would be. But lots of guys add them as they do look nice. I'm far from a "numbers correct" guy as they get and use what I like and think will look and run good.