Is this an aftermarket or factory intake?

It's a factory stock manifold .. there are a fair number floating around. Quite a few '48-'49s (I think) were equipped with them. Maybe someone else knows what, if anything, dictated whether a car got the cast iron or the aluminum manifold.

If it stays around that price, it would be a good deal. Lighter than iron, can be polished and ported easier for better flow than cast iron can.