those blockoffs were cast as part of the intake, then workers cut them out at the bolt holes manually on a band saw, which makes them unique to each manifold.
at one time, those blockoffs were reproduced, and they were made with the intake holes complete and in an oversized square piece.
it was then possible to cut the plates to fit the manifolds you had by placing a line through the bolt holes that matched your intakes.
i only saw adds for those a couple of times, so i'm guessing they are no longer available.
i was lucky with my manifolds, as i got them complete, minus only the carbs, from a 300F my buddy scrapped back in my junkyard days.
i also got the tach drive distributor from that setup.
we almost tore that car in half when loading it on the tilt bed, it was that far gone.
if i had been smart, [never was accused of being that. ] i would have taken a few other items off that car before it was crushed.