Got this beauty from Jim Fielding at the recent TX show.
It's 6.25 inches long, an inch thick, and goes from 2.25"-3 in width.
It's a N'530 on page 10 - http://www.insulators.info/catalogs/lelectroverre/1948/page10.html
Page 11 of the catalog shows some pictures and how they were wired in.
You can also look at page 47 in Crown Jewels April 2003 for information.
Thanks for looking.