AC MK IV Main Tube Bracing?

[westcott]

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

[westcott]

No insider infos or pictures, nothing??
   
   Come on.