The four bolt Holley "teapot" carburetor used on the later Mercury flatheads confuses many folks. I have even heard these called marine carburetors. The nice thing about them is they give us a four bolt intake manifold that will accept some Rochester 2GC carburetors without an adaptor. Unfortunately, doing this eliminates the vacuum advance on the stock distributor, just as any carburetor switch does on a flathead. There were aluminum versions of these manifolds made in Canada. This photo was provided by one of the regulars at the old Classicar Flathead Forum.