The insulator in question is the one on the right. It is shown next to a M-2115 for size comparison. The dimensions are 7.25 - 5.25 X 6.50. The closest match I can find in Elton's book is the M-2260A but the skirt is much smaller on this (5.25") or the M-2342 which also has a much larger bottom skirt and is 1/2" taller. The glaze appears to be Thomas and this was from a line built between 1900 - 1910. Any help is much appreciated and Thanks in advance!

Answer - This is an unlisted porcelain multipart that has not been identified or catalogued previously. Elton Gish is going to issue it a unique M number - it's a new discovery! Thanks also to Chris Hedges for his help as a cross reference to verify it's uniqueness. When was the last time a new multipart was found?