Came across this secondary rack spool recently. What makes it so special is the fact that it's marked with an embossed M in a circle. According to Elton Gish's book It states that this marking was only used by Illinois Porcelain Insulator Company in 1915. This is the first time I have ever found a piece with this marking. I have nail knobs with just the M embossed and a wire holder (Shown in Picture) but never the circle M. There is a patent number 2,834,830 from 1958 that shows a similar wireholder but not from 1915. Truly a great find and glad to have it. They discontinued this marking because another company already had it and they went to the triangle M marking. But you can read the rest of that in Elton Gish's guide book.