Now this is a really cool find. It's a telephone transposition, CD 202, and this is one of the early ones with Fred Locke's patent date on it. He was an interesting character. You can read all about him here: http://fredlocke.insulators.info/
Transposing the telephone lines (basically crossing them ever so often) helped reduce "cross-talk." I'm afraid I don't have a lot of technical electrical knowledge to explain it in more detail than that. But over the years of open telephone wire, quite a few transposition styles developed and usually the indication is that they have two wire grooves separated by either an "umbrella" or (later on) a wide, empty groove.
It's certainly better than the average find. Current PG has these at 5-10.
Thanks for the great info as I am one of those that just loves to recover all things glass while diving. I have 15 or so other insulators all found in river bottoms, I will put all of them in a post after the holiday.