Superboy in "The Cofession of Superboy"
Lana Lang exposes Superboy to red Kryptonite, in an attempt to expose his secret identity. Superboy loses his voice, but still manages to keep his identity as Clark Kent a secret.
Tales of the Legion of Super-Heroes in "The Secret Power of the Mystery Super-hero" *Desperate for aid in their upcoming battle against the space pirate Roxxas, the Legionnaires interview several new applicants. One in particular, "Mystery Lad", volunteers to demonstrate his ability to only one member, Saturn Girl, provided it be kept a secret. She then recommends him for membership.
Voted in, Mystery Lad fights alongside the Legion as they comb the galaxy for members of Roxxas's gang, and vaguely demonstrates his secret power several times. Finally, he realizes that Roxxas is really after him, and leaving his teammates behind, surrenders to the villain. Then he uses his power, that of transmutation, to destroy the pirate's ship, and the Legion captures the villain and his band.
Mystery Lad is revealed as the last of a race whose members could transmute the elements, and who were wiped out by Roxxas and his followers. When the other Legionnaires realize Mystery Lad's real power, Cosmic Boy rechristens him Element Lad. *Source: The Official Legion of Super-Heroes Index #2 - Published by ICG.
(1st Chronological Appearance of Element Lad - goes by the name 'Mystery Lad' for most of this issue).
An untitled Casey the Cop strip.