Multipart Porcelain Insulators - M (Multipart)
When selecting a M-number for a specific insulator, start in the section of the M-Chart that matches the number of shells cemented together. Next check the diameter of the top shell and overall height and look in that area. For example, if your 3-part insulator has a top diameter of 13" and is 11" tall, look in the area of M-3530 to M-3538. If you can't find an M-number that matches, look in the previous group for 12.5" diameter and the latter group with 13.5" diameter. The same variation can apply to the height. You can expect measurements to vary +/- 1/2" or more.