Here is my little JD cable insulator. Really neat, although I do not know the U-number. I tried looking it up but I think I do not have the latest edition to the porcelain books. Any help is appreciated! It is 3 5/8 inches tall and 3 5/8 inches wide. UPDATE: Thanks to David Dahle, Jeff Markwart, and Elton Gish I can now say that this is the first reported JD U-503. Thanks again for all the help guys!