Grisbaum & Kehrein - Milwaukee
The potter that made this bottle for Grisbaum and Kehrein apparently put printer's type together for each batch. The person who did this may have had bad eyesight, was missing certain letters, or was simply not careful when he created the stamps. Most had errors, and there are a number of different stamp configurations known. In fact, one of the hardest varieties to find is the one where everything is correct.
This one is stamped "GR1SBAUM, 8 KEHREIN"