Yeah, but I'm not big into Golden Age spy comics. Lemme ask some friends.

To be honest, I don't remember any comics characters ever using a decoder ring; I only remember them as giveaways to fans.

Does it need to be the main character that users the ring or can it be a subordinate (that is, Kato instead of the Green Hornet, not that that example fits)? Can it be a strip or does it need to be a book?