This is a vintage Armstrong insulator # DP1 also known as CD 155. The antique clear glass insulator is embossed "made in the USA -15 50." along with "Armstrong's DP 1" around the base of the skirt. The clear insulator features a double petticoat shirt with a smooth bottom edge. According to the reference below, DP insulators were coded with two numbers, the mold number, followed by the year the mold was initiated, followed by dots, each one indicating an additional year. Thus, this insulator would be mold # 15, design introduced 1950, one dot adding a year making the actual year produced 1951. Armstrong purchased Whitall Tatum in the 1930s, and was later sold to Kerr. My father found this antique insulator on an old telephone/ telegraph line running along an abandoned railroad track more than 40 years ago. The vintage insulator is in very nice condition, free from breaks or cracks. Embossing is still sharp. Clarity is excellent. there are a few age related bubbles in the glass (See Photos) The old telegraph insulator measures about 4.25" tall and 3.75" wide. http://rypn.org/forums/viewtopic.php?f=1 t=38446