I found this in the March 5, 1921 issue of Electrical World. Lewis Stillwell was granted Design patent No. 34,211 on March 12, 1901 for glaze-weld style U-966. It was made by the R. Thomas & Sons Co. for the 2nd Niagara Falls to Buffalo transmission line. http://reference.insulators.info/patents/detail/?patent=D34211