Hate to shoot down these theories but I have just looked at a 51 Chev and a 52 GMC both with tire carriers and they do not use that center hole. If you look in the Factory Assembly manual (green book) they show the J bolt attaching on the right side. I also do not think that there would be enough strength in that sheet metal piece, even though it is reinforced, to hold up the tire carrier and the weight.
Having said all this I have on my 49 a channel which runs front to rear from the bed cross member which looks like a tire slide would fit in it. And this is different to what is shown in the green book. If that is what it is then it would need a spot for the J hook near the center.
Interesting topic.
