I found these glass rings in an abandoned railway station building in northern Norway, they were probably used to isolate the radio antenna at a time when radio was the only means of communication in these areas. They are about 4.5 cm in diameter, the place of joining the glass is ground and it looks like the screws were mounted before the glass was joined, at least I think so. An interesting addition to the collection. I found some information about them (https://www.nia.org/notins/nons5.htm) where they are described as "awings", maybe this is correct but the ones I have have threaded bolts so they were probably screwed into the wall as antenna isolators.