In SUPERMAN #198, Clark Kent used an x-ray gun to reveal that Clark Kent was really Superman. Really. Not only was it on the cover, it was in the story inside.
Of course, it turned out that "our" Clark Kent was the real one all along, but for story purposes he had to have an out so that he could keep on using his secret identity.
So he reveals that the "x-ray gun" actually projects a Superman insignia on whoever it's aimed at.
In other words, it's more or less the same thing as the "Krypto-Ray Gun", and looks a lot like it, too. But nobody recognizes Clark Kent's clever trick and so everybody is fooled.
Somehow, Captain Marvel never got to play keen tricks like this on his friends in his stories.