A friend of mine dug up this porcelain base for a Fred Locke multipart.. Only about 2/3's of the base is there, and was found at a privy layer that the digger dated to the early 1900's. Privy was in Vallejo, California, which is along the route of the historic guttertop power line from the Colgate Powerhouse to Oakland, CA. built by the Bay Counties Power Company.
According to Paul Greaves and Mike Spadafora, the skirt is likely a 1902 production M-2785 "big crown" gutter or a M-2842a " mold line top ". Various glaze and surface texture characteristics of it make it pretty likely it is from one of these two insulators and not an older M-2796.