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Looks to me like it was in an accident and the frame was pulled back out. You'll want to check it corner to corner and level it and make sure all the mounting locations are on the same plane.
I don't know if its normal, but mine is exactly like yours (I have a '75). I'd post pics, but it would be redundant--it looks just like what's in your pics.
Just Crawled up under my 75 and it looks just like yours to so Id say other than that little rust where it might have twitsted A little It looks like original mine has A metal cover over the area to the back and you can still see the twist in the metal out twards the Lihgts.
Since there are others with the same twist ('75-'76) looks like it's factory intentional. It's interesting that DB's '77 is straight. My guess is that the frame is twisted to match the angle of the fuel tank so the rear edge sits flat. I wonder if they only did this for specific years???? Kinda Funky....gp