Deriving his powers from the elixir of Gingold, he's the Stretchable Sleuth. When there's a mystery afoot, Ralph Didney, with his famous twitching nose, and his socialite wife, Sue, make an unbeatable team.
I don't actually own this postcard, nor the DC Comics Classic Library: Justice League of America by George Perez, a recent hardcover that reprints the art from this postcard set. However, I found a pair of pictures on eBay, and doctored them up to preserve here. I understand it's numbered P6197, and was part of a 14 card set, plus a promotional Batman piece.