Here's a pic of the insulator in question taken along side a CD 156.1 of the collectors album. The wire groove of the 156.1 is aprx. 1/2" wide and 3/8" deep. The wire groove of the unknown is aprx. 1/8"-3/16" wide and about the same deep. The flat area at the start of the dome section appears to be the same with as the wire groove itself. Can anyone identify this CD# now? And thanks to everyone who guessed it was a CD 156.1 from my last posted pics. Maybe this descriptuion and the pic will help with your next guess.