Looks like it could be off a Checca. These were single cylinder 4 stroke made by MV throughout the 1960's they were actually produced in 83 and 99cc capacities, could also be of a 125 GT or GTL again a 1960's 4 stroke single design. The 150RS had that design on the tank but the bottom curved down towards the back whereas yours appears to be flat all the way along. Just found a picture of a 125 GTL which has that design of tank (sorry can't post it cos I dont have a scanner) do the front webs have a bolt hole for securing to the headstock? Welcome to the site BTW.
Thank you very much
Correct, the bottom is flat.
It appears that, unfortunately, somebody has "modified" the tank a little bit :stickpoke
The seller told me, that the tank had originally two bolt holes, one on each side, but he shortened the front of the tank, to make it suitable for a other bike. I will send more pictures tomorrow.