It is thought that the Graybar Electric Company supplied this Dry Spot type insulator for Am. Tel. & Tel. Co. and was listed in one of the early catalogs. These are very hard to find because they would only be used at a line termination.
One unit I have is incused on the metal cap PAT APL FOR. These are found in clear or light purple. Further information about this insulator was found at:
Retreved from the National Insulator Association page "Insulators in Catalogs" Bob Berry at; http://www.nia.org/research/catalogs/text/graybar1_1929.htm
Graybar Electric Company - Catalog 100, 1929 Page 898
Diamond Bridle Wire Insulators
For insuring a dry connection between a bare wire and an insulated wire tapped therefrom. Where a dry connection is necessary to prevent leakage over and around the insulation the wire is used. The braid and rubber portion is stripped from the bare wire which is passed through the messenger-proof cup and soldered to prevent messinger following the bare wire into the protected glass petticoat.
It is used on loading coils in telephone construction and or lead-in wires on telegraph strands.
Price per 100 $40.00
The measurement of these are 3 13/16" tall by 1 3/8" wide at the bottom. The cap is 3/4'" wide. They are found in purple or clear. These are found unembossed or with "PAT APL FOR"