I found a cheap 500E tank, I'm going to give it a shot. Worst case, I can put it up on E-Bay.

So here's the resolution. I bought 1245001549, which is the tank for a 124 500E. And the verdict is...it's perfect. If I photographed the 201 tank next to this one, you'd only be able to spot the difference if I pointed it out.

Sometimes I think it would be fun to find the engineer who designed this stuff and ask him what he was thinking of. The 500E tank is clearly made in a different mold. But it's dimensionally identical to the older 190DT tank, the only differences are in the markings and an inconsequential difference the curvature of the corners. It must have cost a bundle to design the same tank and create a new mold. Was the old mold misplaced? Was this just a parallel development? Are 124's too good for 190 parts? I guess that's why these cars cost so much.

Anyway, if anyone else is in this situation...you can buy the "correct" 190DT part for $202, or the 124 500E part, it's identical twin in every important respect, for $105.