Although it was made pretty clear in continuity that Superman only learns of Krypton in his adult-hood, three years later DC is ret-conning in Superboy #20 (April 1952). In this story, a scientist tries to swindle Smallville residents by having them chat with people from their past. Clark is caught off-guard with a discussion with Jor-el, but it turns out to be fake. Nonetheless, it is made clear that Superboy knows of his alien heritage prior to this point… how?