Morning Kyle. That piece you are asking about is a separate piece of angle iron, that I assume ford used as a brace to strengthen the deck lid. If you look at the pictures you will see it is the full width of the deck lid. It was then waffle head riveted to the inner skin first. You can see where I marked it with blue tape and hopefully you can barely see the rivet heads. There are five total to hold it to the inner skin. Then if you can picture opening the deck lid, they riveted to the hinges with two waffle head rivets each but the other two that look like rivets are actually screws. If you look at post 9, pictures 3, and 5 you can probably see them. When I did the howells deck lid on my green roadster, I added a piece just like this on it. So if you can picture opening up the side seems, and the rear seam, and separating the outer and lower skin, that angle piece would remain on the lower or inner skin.
One more point of information, if you need it. The angle iron that faces up and gets riveted to the inner skin, is app. 3/4". But the one that is riveted to the hinge side is 1". In other words 3/4" by 1" angle.
And you right. You don't see many stock deck lids. I think the farmers s--t canned them to carry there firewood,,,,,,,,or sheep.
Made a couple of corrections. Sorry if you read this prior to my corrections. I kept using outer skin wording when it was really the inner skin.