It's the bottom piece of a cemented multipart, which was likely two shells cemented together (maybe three, but this looks too small for a three-part). With only half the insulator, it's a bit tough to tell exactly what type it is, but it probably would have looked somewhat similar to this one:
http://www.allinsulators.com/photos/Porcelain/M/2200-2299.html#M-2221