Maria - Not sure what that mirror came out of, but sorry to say it's not original (factory equipment) to your truck. Art Deco trucks (1941-46) did not come equipped with inside rearview mirrors, only the exterior mounted ones. Passenger cars did, but they didn't look like the one you have.
That point is neither here nor there though
as it's currently in your truck and sounds like you want to keep it. With that in mind, you might try some weather strip adhesive applied with a tooth pick. Once you dab some into the channel where that bead rests, use a bunch of wide flat rubber bands to pull it into the groove and hold it in place while the adhesive cures. You might have to gently pry open that groove where the bead rests to get the adhesive down into the bottom of the channel. Let us know how it turns out.
