The plate has a nut welded to the back and is spot-welded in three places. There are dimples in the tank just deep enough to make room for the nuts.
I would not recommend welding on the tank (factory did it before it ever had gas in it). I believe even an 18mm bolt will puncture the tank without fairings and such between.
Another user here has since freed his with a spot weld cutter and then soldered a tiny bit of sheet metal over the hole. I tried the same thing and ended up cutting more holes in the tank because the plate would not free even after drilling through the plate.
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