This subject caught my interest, so I did some research. Using the 75 MPC, I looked in the illustration section under the electrical, engine and instrument sections, and then the same in the parts section. I found the manifold, the sender, the wiring (assembly manuals) but found no reference to the adapter. I know it exists, I have one on an engine (a 65 San Jose GT, "A" code, Fastback) to adapt the manifold to the small size sender. It appears to be a brass fitting. My 66 GT, "K" code, Fastback has the large sender.
Jim