It looks like a 1980-1981 CB650C fuel tank.
Couldn't prove by me. I DO have an 80' CB650C that came
without any bodywork at all and I've never really looked at the frame
to see how the tank would mount in the rear. Is it mounted with a
single bolt in the rear or some kind of rubber piece?
Later on, Bill

It is mounted with a single bolt and a rubber damper.
At a first glance it looked like a 650C tank, but I checked the parts diagrams and the tank mazingerzeca are asking about can't be a 650C tank. The C tank have got 3 screw holes for the Honda badge (also the 650SC-Nighthawk have got three screw holes), while the tank up for sale have got two.
And the distance between the screw holes for the badges makes me wonder if it is a Honda tank at all. It could be for another brand, like one of the Kawasaki LTD models, or a 78-80 Suzuki GS550L.