They’re stamped M0408. The cylinder head flanges are doubled and welded together. All joints are slip fit and overlap welded not butt welded.

 

They don't look like Kline to me, if that helps.

 

They look like Cargraphic

https://www.cargraphic.de/en/your-vehicle/for-porsche/for-911/for-9971/for-gt2-36l/exhaust-systems-and-components-for-porsche-9971-gt2-36l/sport-manifold-set-for-porsche-9971-gt2-36l/cargraphic-manifold-set/

 

Wow, thanks for that clue. Cargraphic it must be. Your link show everything: doubled and welded head flanges, slip fit overlap welded pipe joints, and turbo manifold flange design all there. Many thanks!!