by Robin Plewes » Sat Aug 12, 2017 8:13 pm
This is way late for a reply, but my thought is that this mark was caused by another Brookfield embossed insulator bumping into this insulator when the glass was very hot and still soft. If a cold insulator happened to bump against the soft insulator in the kiln, then the "Brookfield" embossing would act like a stamp and make an impression in the soft glass.
The "3" is probably most of a "B" in a Brookfield embossed insulator.
Just a thought ............ Robin