Thanks for all the feedback. My assumption of the spare set of manifolds was correct that they are from a P30.
As for the P35 manifolds I have a theory. Early production the manifolds were bigger volume but smooth on the outside. Exhaust heat caused distortion in the manifolds. Mine have a good amount of distortion and took quite a bit of sanding to make the cylinder flange flat again.
Later production used the ribbed manifolds that had more rigidity and heat disapation.
Just a theory. I'll have to compare serial numbers.
I will try and take a picture of all 3 types when I get home and post them up.