Source: Superman vol.1 #148 (1961)
Type: ImpostorsRemember the time Mr. Mxyzptlk changed his name to John Trix so that Superman wouldn't be able to send him back to the 5th dimension without first figuring out his new name? And what does that have to do with super-convicts. The answers to those questions are, respectively, probably not, and not very much at all!
The short story has Mxy do exactly that (5-dimensional laws make the name change effective for only 75 days), as he goes on a veritable rampage of shenanigans - changing famous artwork, dunking the mayor in glue, animating manhole covers, and yes, changing an entire prison's worth of convicts into Supermen who could now easily escape!
But don't. They put in two appearances. The above panel, and the cover. THEY MADE THE COVER?! And Superman doesn't even deal with them. Instead, he goes to the source: Mxy's new name. To get it will require a convoluted plan and a certain measure of stupidity from the villain. Here goes. Superman gets the Emergency Superman Squad - a tiny Kandorian super-team which I really must get to some day - to animate a Mxyzptlk puppet. Mxy2 says he's the real Mxy, but Mxy1 won't be shamed into something stupid by what he assumes is a robot. To prove he's not a robot, Mxy2 says his name backwards and disappears from view (the Superman Squad simply breaks in and out of the time barrier, just like that). So to prove he IS the real Mxy, he says his new name backwards and off he goes. And now for the creepy reveal:
So if you feel cheated that these one-panel Supermen got their own post in Reign while perfectly good Supermen still remain ignored out there, well, imagine how the kids felt in 1961.