Why are my gas tanks changed?

I have the carpet out in my trunk to investagate a gas smell and discovered I had custom made gas tanks made for my car in 1982 (on data sticker that is on the tank) the car is a 1981 so they were not wore out was it for the impact bumpers to federlize? if so do others have the same thing? my car has what appears to be beams installed in the doors by the same German CO. who did the other federlizing. is that common also?
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Thanks guys for the help.
Serial number is 40501. I belive it is a 1981 car.
From what I have gathered, this car was purhased new to import into the US. It came through Dussoldorf Germany where it could then be declaired "used" then it was importated to the US and Federalized in 1982, the EPA and DOT release letters and the tank sticker etc. all have 1982 year date. Why the tanks needed to be custom made is a real mystery? I do know the Federlazation modifications were far from standard but this seems a bit over the top for import reasons, is there another? that is why I was hoping somebody else who had a car brought new for importation could tell me it was done on their car also. These cars do contain a lot of ??? must be part of the fun.

 

I find it hard to belive that the fuel tanks failed and had to be changed when the car is only a few months old. If so would they not change the failed tank? did both fail after less than a year and need to be changed? And the 400I was still in production when the tanks were made in my car. Good idea, but I think something else is going on here.