Curious about your interest in this particular detail. The GM Parts Master doesn't shed much light on the construction of group 0.289 . . . save the 29-33 which is described as 'steelbestos' . . .
Old Car Manual Project . . . likely a composite including that infamous material.
Shim steel gaskets are not very user friendly. They assume new construction with straight and smooth mating surfaces. They also required special coatings. From what I have read the builder would spray the gasket with a special silver paint and then dust the wet surface with a fine aluminum powder. Seems awfully time consuming for an assembly line procedure . . .
