Just got my 1st triumph, 1st thing i want to do is clean it and ride it... now it's still in process on disassemble, clean and reassemble it back.. i want to feel riding it 1st
that is the Crankcase breather. It is attached to the fender. You have no fender.
You can do this to make it look good and keep bugs from crawling into your primary.
__________________
"Thanks to denial, I'm immortal" -Phillip J. Fry
if the bike is a 79 then it's the correct nut and should have a plastic cover.
I replaced mine with a stainless one Shrugger shows what I intend to do with my primary vent. But the original didn't go to the chain, as the E vented into the RHS air box. If mounting filters as standard, remember to seal that hole