Boy, Scott. I don't know what you have there. It looks like Chevy used several different types of u joints in the 48-54 pickups, from what I was able to see on line. None of those look like yours.
I think you should start calling some of the Obsolete Chevy type places and run it by them. It seems those yokes are hard to come by from what some of the forums were saying when I looked a little tonight, and also big $$$$$$$ when you find them.
Good luck, let us know how you make out.
Don