I am hoping that the land of ICON can help me with a question. I picked up a 162 H.G. Co in ice blue, it has a circle on top of the dome it looks like the mold was machined out and plugged, and it left this mark, and there is not any hint of a number or letter in the circle. I have been told that another very close looking insulator has the same circle on the dome. Is this something that should be added to the guide? I hope the picture is clear enough, Thanks, John Mesmer.