A nice little archaeological find in DC’s Tales of the Unexpected #16 (June 1957): Jack Kirby brings us a short story called “The Magic Hammer”. In it, a nasty prospector discovers Thor’s hammer, Mjolnir, and uses it to summon rain and build himself a crooked fortune.
Eventually Thor comes to claim the hammer, telling the story of how Loki stole it and lost it. While the character designs are very different from Marvel’s Thor and Loki, the hammer design is basically identical.