Quick release cap - Interstate

It's held on by a thing called a roll pin. You tap it into one side of the cap, position the cap and then tap the pin all the way in. You need a pin punch to knock it out if you want to remove the cap. Best to protect the tank paintwork around the filler cap when fitting or removing.

I'm pretty sure it is welded. 

Welded on both my Mk1a and Mk3 850 tanks. Had to change filler cap on one of the bikes a few years back after warning given on annual MOT test - easy to remove and fit new one as described above.

My brother, me and some mates used to help a friend with his hot rod Mini racer.  The petrol tank needed welding but my brother was a bit nervous about it so he left it under a running stream for a week to get all the petrol out.  When the day came, he started welding a bit gingerly and Andy who owned the car hit the metal workshop door with a hammer.  He went dashing out of the door while the rest of us rolled helplessly on the floor laughing.  Workshop humour................