The CD 158.6 has an interesting slotted pin hole. You can see this here and in the next picture.

There is a Utility patent 306,147 granted to Lawrence B. GRAY and Joseph HAM on April 28, 1884 related to a collapsible mandrel for forming threads.

http://reference.insulators.info/patents/detail/?patent=U306147

A second Utility patent 306,146 granted only to Lawrence B. GRAY on the same date describes a different variation of a collapsible mandrel

http://reference.insulators.info/patents/detail/?patent=U306146

Since the embossing on the base of the insulator includes

AND HAM PATENT

one can assume this applies to the former patent, as that is the only one assigned to both Ham and Gray. One might guess that since the patent drawing is attributed to both men, the missing embossing could include "L. B. GRAY" ... making it read "L. B. GRAY AND HAM"