What is that product under the saftey glass?

[jroberts500]

I am about to remove the safety glass from a roundie(original '63 RCA hec). I know most everything about the process but I wish I could obtain whatever that material is to put it back properly. Without it I guess I will do a bead of silicone with four spacers. I am concerned it will be visible that the glass and safety glass are seperate. I am guessing that stuff comes in sheets and is probably put on the tube hot and then the safety glass must be pressed on.
I bet somebody here knows!
I am still trying to figure out how to get photos of three old sets I am restoring.
Thanks and have a nice day! John