The publication date rules, Jim, not creation, and I can't believe that nobody ever noticed a 1934 Superman comic strip drawn by Russell Keaton in the papers. If it was never actually published (for example, if it was circulated among fans after finding it at an auction--as I said, I have no clue how these came to light), the copyright would be Life+70.
The only situation I could imagine where there might not be copyright is, if this was produced by the Siegel estate as part of trying to reclaim the assigned copyright, its first publication would be in court records. The records generally are (I believe) generally considered public domain, because case law is kind of important for people to see, but I don't know what the rules are on evidence, and I don't know if that's how the strips came to be known.