I bought one of these to fit a Dyna tank to my Sportster & was wondering if anyone knew how it mounted.

 

It doesn't look like an FXR mounting, Dynas are FXDs! The FXR tank has an integral flat flange through which two bolts go, it doesn't use anything like that bracket. You may need a Dyna owner.

 

That could be an adapter for the front tank mounts. If so, it looks like it would be plug welded to the frame backbone. Measure the distance between the two outside surfaces of those mounting bosses. If it's around 3 1/8" then it could be an FXR front mount. As Graham said, you still will need something to adapt the rear mounts of the FXR tank. That uses a curved mounting plate with two bolt holes about 4" on center.