Produced by the Hemingray Glass Company, a prominent American glass manufacturer founded in 1848. These insulators were a common glass insulator primarily used to support and insulate telegraph, telephone, and electric power lines on poles across North America, from 1921 until the 1960s. Often repurposed for decorative items like paperweights or converted into lighting fixtures. Approximately 3 1/2” high, 3 ¼” diameter. The markings are: 40-42::::, 51-42::::, and 39-42:::: All state Made in U.S.A. $23 for the set of 3.