Probably the same stuff used to secure the door glass to the brackets which we all know wasn't very good since 69 glass is renowned for falling out.
I got a puck of 2 part epoxy from an auto glass shop, but never used it since I switched to bolt-in glass.
It has a dark center circle of catalyst, and a lighter outside ring of glue.
Try your local auto glass shop or body shop supply wholesaler for a glass to metal adhesive.
The procedure might be detailed in the assembly manual, but I don't have that book, so can't help you there.