The Electrical Contractors and Maintenance Co (EC&M) of San Francisco provided electrical equipment in California in the 1870’s.  The EC&M insulators were used on telegraph lines extensively in the west, yet remain relatively scarce.  This aqua colored one is by far the most common, but they also come in other colors such as green, amber and cobalt blue!  Some are so valuable that fakes have been made and sold to unsuspecting hobbyists.