I've had '51 Fords since 1957 and don't know what you mean by "skid plate". Granted, none of them were convertibles, but I can't see such an appurtenance being convertible only. Can you clarify what you're looking for?
No, I think "nelsb01" is referring to the sheet metal plate behind the bottom of the radiator. I bolts to the frame rails. It is almost flat but has some creases in it to add rigidity. It also has a hole in it for the radiator petcock to drain through.
I can see why some lazy mechanics might remove it and not put it back. It makes accessing the water pumps and lower hoses from below difficult.
No, that's not the skid plate. The skid plate is made of thicker steel and bolts to the bottom and top of the frame at the very front. Its function is the raise the frame (car) if the car encounters an obstruction, such as a large rock. I will try to get a photo of the skid plate.
To me, that looks more like an air shield for the radiator. To help maximize flow through the radiator with a shroud. It looks to thin and higher than the front cross member to act as a skid plate. Anyhow, I’ve learned something new…..Thank You! None of the Shoeboxes I’ve had, have had one………….Mark