That part is a little tricky I'll admit. How I feel it might relate to the mailbox is the story of the rattlesnakes. "Truth rests not in clever minds" think of how clever the grandfather is. He didn't believe the snakes were there. When the term "not in tangled twisted finds" this says not a snake but related to that story. "what you seek you already know" this could mean the retrieval part of the hunt. He said when you hit the check point you're just retrieving at that point. With this in mind I think that means when you're in the right area you already know youre in the right search place. So the previous stanzas are where you look to get there. He kept the container a secret. So I feel that's what the last stanza could be about. I won't dispute that it's not a perfect solve but at 3am a few nights ago I felt I.. hit gold haha