I like stupid questions - it's possible many people may want to ask them but are too Embarrassed. So I look at it as if I'm helping others! (And me)
My filler / caps are identical so I'll take both off to see if one is different or actually a breather / vent. I haven't got any info on this engine, it's got a few upgrades and lights the tyres in 2nd and 3rd, which I'm happy about of course. I think the rear diff ratio is standard but with this 400+hp, it may need changing to stop lighting tyres and because 3000rpm is about 60mph which feels too high. I'll probably post a few questions on that too.