I got this CD 233 from the same PA junk dealer as the CD 103's I posted yesterday. I picked thru quite a few insulators the guy had and I noticed the odd marking K13•39 this one has on it. Maybe it's some kind of code put on it by a worker at the Pyrex plant? Update: Michael Spadafora shared with me that the marking was done with the pyro-glaze method while the glass was still hot and that's how the marking has survived all these years since 1939 maybe?