I found the information you were looking for Leon. There is a book called "Baseball Goes To War" and in it the author mentioned the 1943 Brooklyn Dodgers War Bond Campaign. He says that in April of 1943, Branch Rickey agreed to attend a Sunday Game and break his commitment to the Sabbath. The reason he did it was to attend a US Treasury Department Ceremony honoring the Dodger's Spring Time Offensive. Apparently the Brooklyn players would walk into the stands and sell bonds to anyone attending the games that Spring. According to the book, the Dodgers alone raised $180,000. The card you have must have been one of the ones the players had given out to fans during the games in April and was just never filled out.
See if this link works, third paragraph:
http://books.google.com/books?id=xri...20bond&f=false