D.E. Whiton Four Jaw Chuck help

So while shopping at my favorite place, the local scrap yard, I came across this ancient four jaw chuck. It is marked The D.E. Whiton Machine Co. , New London Conn. U.S.A. Patented Sep 13, 1887. Overall, it is very nice condition with the only issues is that it is quite grungy and that the corners of the sockets of the adjusting screws have some cracks. I would like to weld those up but have not been able to figure out how to remove the retaining plates that hold them in! There are set screws on the back that lock them in place with half the threads in the retaining plate and the other half in the chuck body. The screws came out easily enough but the plates refuse to move even when subjected to careful tapping with a brass rod and hammer. I've been soaking the parts with PB Blaster and Kroil but still no joy. The backing plate has a thread that is approximately 1" in diameter that I would have to change if I wanted to use this on my SB or Logan lathes. Anybody have any ideas how to get those retaining plates out? I'll see if I can pictures later today.