I found these last month near an open mine entrance that goes back to the 19th century not too far from where I live. I'd say these are probably from the early 20th century, they can't be much newer than the 20s, that's when the coal ran out and this mine shut down. They are threaded, but stuck on the metal spring type threads. I've never seen or head any mention of threaded procelain mine insulators that look like these ones. Anyone know if these have a U# yet?

Update: These have been assigned U-92 by Elton.