"The diameter of the large hole in the base and the manifold match so I am assuming it’s for a 235, not a 261. "

Why that assumption?

I have "interchanged" 1954 235 carburetors with later 261 carburetors.

https://www.classicparts.com/1954-62-Carburetor-Base-Gasket-L-6-235/productinfo/81-210/