In The Brave and the Bold #3 (October 1955), Jon the Viking Prince rescues Captain Olaf from Baron Thorvold using a hammer he claims he finds in the sea and claims it is Thor’s. Unfortunately it is just trickery and not really Mjolnir.
[Yup, three years after starting this blog, and over 8 months since pausing it, I decided to fire it up again! My prediction last year that I’d be in the Marvel Age was way off but I hadn’t accounted for taking a 8-month break :-/]