This was found in O-B's 1921 Catalog No. 18 on page 528 (full catalog available for download on google books). The page mentions this insulator's design being capable of handling 2,400 volt DC operation. It also mentions the well known line that was using these in Michigan, which you can read more about in the summer 2014 copy of Drip Points (article starts on page 14). http://www.nia.org/drippoints/current/holding333/Summer-2014-web.pdf

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