When did calling Superman "Kal" become a thing. It makes sense in the Silver Age. Through a good portion of that era, he had some memory of Krypton or else all his communication with AI Jor-El. I recall a Superboy story where he didn't like Jonathan Kent calling him "Clark" because he wanted to be called by his "real name" when they were in private.
I haven't read 52, but maybe it makes sense there, too.
But it seems like I've seen Diana, at least, call him that in a post-Crisis era. I remember being surprised by it. But my memory may be faulty. In any event, it really never made sense to me in post-COIE era because he'd never lived on Krypton, really, or gone by that name. He thought of himself as an Earthling...some of the time, anyway. Clark's the real guy (version I prefer).
Anyway, I was just wondering if anyone knew where calling Superman "Kal" began, and how widespread it was (in regards to number of people using it or years used)?