I found this insulator at an antique store today. I bought because I knew that the threadless insulators are older than the threaded, and it was a great price $5. It's embossed on the front S. McKEE & Co. The insulator is chipped at all, but there is a white residue and a few small circular impact fractures that didn't chip off. I was wondering if there's an easy method for cleaning it without damaging the glass? Also, I'm curious as to rarity or value, I can't find a price online. Any help is greatly appreciated. Thanks, Joe