I recently acquired these insulators pins. Both are in remarkable condition. I believe the red stained side pin was for an EC&M telegraph line. Can anyone confirm this?

Answer: Although EC&M's have been found on side pins like this one, so have other types of insulators. My EC&M's do not screw onto this particular side pin. The insulator threads along about a third of the pin threading before it stops. Other collectors have identified characteristics of side pins that are closely associated with EC&M's. Those characteristics include: a beveled top; a narrower diameter threaded shaft; a groove around the full circumference of the pin at the base of the threads; a longer shaft below the threads; and a truncated tail. Nail holes should be square, not round.