The OE didn't have fiber gaskets with the metal valley pans... the raised embossing does the sealing around the ports. When new, the castings were straight, flat and smooth... thus, no need for the additional fiber gaskets (and saved $$ to the assembly of the engine... always counting pennies!).
The fiber supplementary gaskets were OE only with the '69 aluminum six-pack manifold... due to possible warpage and porosity of the aluminum mfld.
However, there is no reason not to use the supplementary fiber gaskets... they are offered by Fel-Pro (and other gasket mfgrs) as an option. Older castings/assemblies will likely benefit by having these installed.
(I was a former Fel-Pro aftermarket engineer for many years.)