Got it open this morning. Had to try close combos and do wiggling but it finally went from 3 to about 11 and opened.
And of course you're all curious what was in there. Nothing hugely valuable, just old equipment manuals. But to Dad those were gold because he was a logger and did his own mechanic work on his equipment. I really miss him.
Thanks for all your help, I've learned a lot through this process.