What it looks like you have here:
is a Locke M-4321, made at the Victor, NY plant. These were listed in Locke catalogs, from 1907 to 1916, and were used quite commonly, on 60kv lines. So, it is definitely an old piece. Checking the value (my book is a few years old) the insulator seems to go for anywhere between $75 to $100. The colorful glaze though, would probably put it at the higher end.