Found on an earlier power line with suspensions of similar vintage. Its a nice swirled grey colour, not a modern skyglaze, which although it didn't show up well in the photo has many blue dots all over it. This is not a flat disc, has 3 petticoats on the under side.

After further research into the line these were used on, it would suggest that these were produced in 1910 by the Ohio Brass Co. for the Hydro Electric Power Commission of Ontario. So far I have only found the remains of others smashed for their metal caps. This particular piece was likely in service until the late 1980's. Most others were replaced by the early 1930's or even earlier.