How about one of these? I looked for one for years. It is a filter that goes on the thermal vacuum switch in the thermostat housing. I know their were several different applications, but mine is on a '68 6 cylinder.
John
DING DING DING!!!! We have a winner!!!
Per the '75 MPC, it is a "Filter - Distributor Vacuum Control Valve". Service part number is C8AZ 12A120-A, no markings on part other than a "2" on top. The "2" may represent the mold cavity, or it could be a reference to the bottom port on the valve, which is also labeled "2". John, does yours have a number on the top? By the way, My guess is that the chamfer on the top is to clear the hose on the middle port of the valve. I haven't tried it yet to confirm.
Mine is black plastic and John's is white or maybe translucent? John, do you know the origin of your example?
I intend to take accurate measurements of mine, perhaps I will look into reproducing it since they seem to be fairly uncommon to find. Probably because they just press onto the bottom port of the valve - seems likely to vibrate loose.
I've attached a diagram showing usage for MY application - there are numerous others. I've also attached a page from the 1968 preliminary shop manual that has the best description I've seen of the filter, and instructions on how to clean it.
Thank you Charles for fixing the topic! While this thread was awaiting repair, I have since run across a few more photos of filters. I will upload those very soon.