I have had my H for along time and never had any problems with that O ring. There is no reason to unscrew the air cap. The TCP site has a Paasche parts and accessory page and shows where that O ring goes.
Thanks for your input guts. Hobby Link has the 2 Porsche kits on clearance for $12.79 and $13.39. I think that is a pretty good price. On my Christmas list they go.