The Whitall Tatum CD 154 is probably the most common purple insulator found in the United States. They were used everywhere and in great numbers. In the modern era, though, they aren't the easiest units to find on poles anymore. These ones are from the extreme eastern section of the Baltimore and Ohio Railroad Wheeling and Pittsburgh Subdivision. Steel mills from the nearby Monongahela River valley covered all the insulators in heavy soot where they were found. I had to stand directly under the pole and look up on a sunny day to pick out the purples from the aquas and straws.