The Pittsburgh M-3930 is an insulator unique to Seattle City Light's transmission line from Ceder Falls to Seattle. These were installed in 1909 when a second independent 60kV circuit was built. The first line built in 1904 used large Muncies with glass bases until 1907 when it was upgraded from 40kV to 60kV with similar Thomas three part multiparts.

These insulators were still in service up into the 1950's when the two lines were replaced with a single 115kV line. The two photos below were taken in 1953 and show both lines in service along the main Ceder River municipal water line to Seattle. In one, you can definitely make out one of these Pittsburg pieces in service.

Apparently they use to give them away at City Light surplus for people to use as flower pots until the supply ran out...

These use to turn up in people's yards in Seattle but I have not herd of one surfacing in many years.

Here is what the transmission line looked like in 1953 when these were still in service.[id=795102730]