So anyone know what this is?

No air tank, if you remove the cap you can pump up the air shocks through that fitting.

Look at the second pic, it looks like an air line running around the rear of the gas tank.

Problem is, when running air shocks (which I personally don't like), they should be plumbed independently so when cornering the air doesn't push from one shock to the other with the weight transfer.

Ah good to know, and thanks very much for the info!