Help Please with Mystery Part

[1967 DeVille Convert]

Greetings Everybody,

I recently purchased a 1967 DeVille Convertible with a 429 engine. My mechanic discovered a coolant leak and the part is rusted. Not only is the part hard to find, but we don't know the name or part number for it.

The part inserts into the top of the water pump and screws into the bottom of the thermostat housing.

I ordered a Chilton's guide for 1967-1989 Cadillac's and hope that this part is identified in the catalog.

Does anybody here know what this part is called, its ID number, and the best source(s) for finding this part?

Your help is much appreciated!

Thanks,
Adam

[ewill3rd]

I couldn't tell you much by that description, got a digital camera?
Photos really eliminate all doubt.

[1967 DeVille Convert]

I attached a diagram of the part in question. The "?" in the diagram identifies the part we are trying to get the name of and locate.