Back in 2009, Richard Case showed a poor copy of a couple of pages from the 1895 Metropolitan Electric Co. catalog that shows a glass knob with fingers. Here is a high resolution scan of that catalog page, which shows the "Stuart insulator". Jim Colburn has a nice example of this insulator he obtained from the collection of Bob and Phoebe Adams, which is the only known specimen [id=220499954]. Darin Cochran and Roger Lucas found a damaged smaller version was found in the Hemingray dump [id=215126692].
A detailed view [id=330608720].