On some inside pictures from the Autokraft plant you can see the MK IV main frame still in the jig without paint.
A closer look shows grinding points on top of the main frames in equal distances.
It looks like something inside like a brace made from flat material was welded through the tubes and grinded afterwards.
Is it really a brace going through the whole length of the main tubes?
Is it flat material or anything else?
My car is actually sitting on a small scissors lift to replace all suspension rubber caps and the doors open and close without any problem.
The bracing would be a explanation for that kind of frame stiffness.
Uwe