I just noticed a lot of uncharacteristic rust on the seat hinge part of the frame. As if someone did some welding there, perhaps to fit the F seat a little better/closer to the tank.
Some people shouldn't have access to powerful tools, (like toddlers with sledgehammers). 
I guess I just don't appreciate frame hackers. 
Cheers,
It appears that the PO had to move or replace the stock seat mounts. The weld looks hideous. 
Not that i could do any better.
Well then, that "repair" could be costly. The F seat you have is nearly trash. And now, the correct seat won't fit on the bike as it should. And neither will a good condition F seat. Check the rear of the frame cross strap arch. This normally caries the load of the seat and there should be bumpers under the seat where this contact point is made. Moving the F seat forward disengages this load transfer point to nowhere functional.
It's gonna be a challenge to correct the seat hinge without a "model" bike to refine/determine the original position in all 3 dimensions.
If your welder is "that good", he'll likely be costly. I can probably provide some dimensions for you, if you wish to restore. But, hacked frames are the domain of cafe racers, rat bikes, bobbers, and choppers who deliberately try to disguise the machine's origins. It can never be "original" again, but it still can have functional utility. Depends on your goal for the bike and how much you are willing to invest in the "hobby".
Cheers,
