This is an example of the "CW" style insulator, which had a copper thimble and strap which would connect to the pin next to it. This was to prevent cross talk on high frequency carrier lines. This thimble allowed the use of a wood pin, rather than a metal pin that was common with the CD 128 carrier styles.

For more information, lookup "CW" in the glossary at http://www.insulators.info/general/glossary/