This insulator came out of a large collection I bought. While inventorying it tonight, I noticed a small stamp below the embossing on the front. It is a star with "Ir" in the center. I was wondering if anyone knows the origin of it. I was thinking it might be the initials of a past owner who wanted to mark all their insulators, or perhaps it mean Irradiated, but the insulator is clear in color. Seems to me I read that some types of irradiation wear off over time in sun or whatever. Anyways, if you have any idea, let me know. UPDATE: Found out the star has HT in it, and was sandblasted on to indicate "Heat Tempered" glass.