Check the long casting number on the intake manifold. To find it, remove the air cleaner housing. This long number is located on the top surface of the intake manifold, and is near the front of the carburetor. The proper intake manifold for a 1961 Thunderbird will have casting (part) number C1AE 9425B. Behind that is another shorter casting number. That number is the actual build date of the intake manifold, and it should predate the build date of the car. The 1961 intake manifold also has a large pipe plug in a boss just in front of the longer casting number. This is where a brass fitting would have been installed as part of the special PCV system that was installed on California DSO cars. The correct 1961 four barrel Ford carburetor has a series of six small holes at the leading (upper) edge of the choke plate.