Fuel filler cap seal

[Adrian Clark]

Hi everyone, I got pulled up at the last race meeting with the 928GTS, for having a missing rubber seal from the petrol filler cap. Intrigued I checked all my 928’s, and none of them had a seal on the inside of the cap?
So looked on PET and there is a seal, part number 92820132702.  So just got a couple and surprised to see that the seal doesn’t appear to fit inside the cap thread, but on the outside (just below the red outer cover). 
Never seen this before!  Can anyone throw any light on this?
Cheers
Adrian