Here is the other side of the CD 732. Whatever it is that is embedded in this threadless insulator is one and a half inches across. I assume it is firebrick, .........but the more you look at it you begin to wonder. Could have possibly came from Roswell, or possibly Stephensville, Tx. whereby a piece of a UFO got trapped in it somehow. Not sure. This insulator was in Ross Baird's insulator collection for many years until he let it go recently, and I was able to purchase it at the right price. Thanks again Ross.