[BillyCoxDodgers3B]

Usually, the only people to refer to a person using a monogram is the person who the monogram belongs to or, once in a while, a biographer of that person. And, of course, if the person is actually well-known by their initials (such as Yves Saint Laurent, Hunter S. Thompson, etc.).

I am convinced that it's Abbaticchio's writing. Attached please see my findings using letter-by-letter comparison.

Perhaps it was an old photo that Abbaticchio found years after the fishing trip and sent to Wagner in the 1950's. That makes sense, as Abbaticchio maintained correspondence with his teammates, and this also explains how the item originates from Wagner's estate.