If I look at that picture intensively my guess would be that the tank is damaged on the edge ant the chrome trim covers it (almost).
Probably a not so careful try to get the tank on the frame (lifted to high trying to find the rubber caps on the frame and making sure the clutch cable lies on top.)
I am very curious indeed what the tank looks like attached on your Bike Seabeowner.
As soon as you have a picture....