Here’s what little I know about the Knox split tree telephone line insulators you found.

Knox was but one of at least 16 different ones of this type used by the U.S.Forest Service and other forestry agencies during the 50-year era of the single wire ground return telephone system. It spanned over 65,000 miles through the woods, connecting 8,000 fire lookouts with their dispatch desks at ranger stations all across America, from 1910 until the development of radio in the 1940s. A few miles of lines still are used in the Wilderness areas of Montana and Idaho.

White porcelain ones like yours by Knox were made until around 1929. The first were 8-sided, equal interlocking halves, tied together with a double wrap of #9 galvanized steel wire, tied loosely to a 4” staple about 20’ up a tree every couple hundred feet, leaving a slight sag to allow for the line to survive a wind-fallen tree. The line would stretch tight in as many spans as needed for the tree to fall to the ground, without breaking the #9 wire.

The idea was first put to use by USFS ranger William Daughs in the St. Joe National Forest, Idaho in 1911. He carved the first ones out of thick bark from Douglas fir trees. When his idea caught on, the USFS put it out for bid, and numerous companies soon provided their version. Knox was probably one of the earliest.