Score another for philately!
It was mentioned in one of the 1967 TV episodes, and confirmed recently at New York Comic Con by the comic books’ current co-editors. You can read the story here. The episode is Episode 51, second season, “A Piece of The Action,” and you can see Bruce and Dick examining stamps, starting at 2:47. One place to watch the episode is here.
The VSC radio feature on this is here.