Doesn't hurt to check ebay occasionally for similar models. You may find some info out there. I've seen snap on boxes that are similar to yours that were made in the '70s.
Is there a way to confirm like a place where they would have put the actual maker...It all seems correct to me...Looking at the bracket behind the MAC emblem all looks factory built.
Just saw a similar box on the vintage tool box site. It was a Mac had a bigger caster and a little different handle on the locker but other than that it was the same...they said it was built in the 60s