I found out it is part of the cover and can be removed as the resin used to bond it can crack and cause vacuum issues allowing air into the engine.
If it's been removed that's probably why.
The only solution I had to get one back on the car as to buy either a new rocker cover or buy a 2nd hand.
I even got my local TPS to get me a cover in to check the filler neck for part numbers but never had anything on it.
Its a pain for sure, the R8 oil cap isn't seeing much light!
@MWright