I found some infomation on those headers in the "How to Hop Up Chevrolet and GMC 6 cylinder Engines" by Roger Huntington copyright 1951.
On page 94 is a picture of these headers with this discription,
"The ultimate in exhaust gas flow refinement is obtained by using the Clark racing-type double exhaust headers in conjunction with dual lines and straight-through mufflers. Complete sets in this class sell at around $70, and tests show no appreciable back pressure up to peak rpm.
Those headers have been sold BTW.
