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Hemingray No 40 blue glass insulator with 5 on top, weighs 1 lb. 5.5 oz. 4" x 3.75"
Antique Hemingray No 40 Aqua Glass Insulator 1910 to 1921.
CD 152 Hemingray No 40 in c. 1910, an experiment with AT&T to produce a new standard insulator resulted in this design being selected. It was produced in mass quanties until 1921." (From Hemingray.info)