It is most likely PBR. Through the ages and by most traditions, the largest letter in a monogram is the surname and the smaller letters the first and middle given names.
With a personal token such as this, who knows. I still think PBR is most likely the recipient's initials but it could be something more symbolic. For instance, a couple who married in 1824, last name beginning with R, with P and B the initials of each person's first name, letters intertwined to represent the union. Just idle speculation though :)