These three insulators are known to have been designed by Ralph Mershon. Early R.Thomas &Sons catalogues listed Mershon's designs with catalog numbers that ended in " M" there was a fourth 6" wide unipart (presumably a glaze-weld ) in the 1904 Thomas catalogue which had a number that ended with an M but it has never been reported by collectors. The U-945 on the right is the only one that comes with a Mershon marking. all three date from the 1899-1904 era and have dry process inner skirts .