Someone asked me what this is. Says it has a standard threaded pinhole. I question that it is an electrical insulator in the typical sense, because there is no wire groove. The person said he got it at a National many years ago...
Thanks for any help!
===== April 2009 update ======
Amazing... Take a look at this patent.
The piece does have a standard threaded pinhole. Mr. N. R. Woodward has assigned it CD 207.2. He considers the top and bottom part of the same insulator, much like the CD 207, which it's threaded top that screws on to the bottom. Thus, the combined unit gets a single CD number.
The bottom "half" of this insulator has not been found.