Oh, so that's what that piece is for. I have one in my empire of parts.I just didn't know what it went to.
They are very hard to find and they are often missing on the 54 & 55 1st series trucks. Unsure why. The metal "stand-offs" that go on the backside are also very hard to find. I recall seeing someone was working on 3-D printing them.
That piece is required to direct the defrost air to the windshield. ( wonder what the correct name is for it )
So, point being - you could probably sell the one you have for a good return on your investment.