Slug embossing was used to remove old embossing on an insulator. The old embossing was ground off and to do so more of the metal of the mold had to be removed around the embossing as well. The new embossing was then embossing inside the "band" aka slug plate.

Sent to me by Craig Boehm showing exactly what slug embossing is. Also see this one too [id=47167688] Thanks to Brent Burger, Robin Plewes, and Craig Boehm