Dating whitall tatum glass insulators
Corning Glass Works in Corning, NY developed the earliest versions in the 1930s, embossed with their PYREX trade name.
Bell Telephone used these on local and long-distance high-frequency lines up through the 1940s.
Hemingray changed the style number to 45 (and kept making and selling 42s) but Whitall Tatum phased out the old style No 1 and put the number 1 on their new version.
We can date most Whitall Tatums by the logo, usually on the back near the MADE IN USA lettering.
The Hemingray 42s are probably one of the most common insulators made.
Both Bell Telephone (AT&T) and Western Union adopted this style as standard for their long-distance telephone and telegraph lines.
These are typically from local phone distribution lines and I'd guess your example is from the 1930s.If they have a WT triangle logo, they were made by Whitall Tatum prior to the 1938 buyout.If they have an A in a circle, they were made after Armstrong bought Whitall Tatum in 1938, but they kept using the WT name for a few years. INSIDE ONE WITH A 44, ANOTHER WITH A 32, Another Light Aqua 19, White glass Whitall Tatum No.3 16 & 1 more that isn't pictured Pyrex clear has KLL. Whitall Tatum No1 79-46 A INSIDE OF CIRCLE 2 Dots under & 1 on upper left...,next one 76-46 A ( SAME THING BUT TWO DOTS ON RIGHT)...,68-46 A Dot same as first...,67-46 A 1 dot on upper left 2 on rt & 2dots under...,60-46A SAME DOTS AS THE FIRST..., Whitall Tatum Co TRIANGLE INT.