It looks as though somebody was after minimal overhang.
I think I would see if Bison will sell the missing part. They sell other chuck parts, so they may sell that one. That would be the simplest solution. Otherwise, I guess you could buy a 6" disk of cast iron from a place like www.mcmaster.com and make the part.
You may also want to make the threaded adapter plate to specifically fit your lathe spindle nose. I bought a Bison 3-jaw with a 1 1/2-8 threaded backplate for my lathe and found that it was a "rattling good fit." I suppose it's because a generic threaded backplate has to fit every 1 1/2-8 spindle nose ever made. I ended up boring out the threads, Loctiting in a cast iron plug, and reboring and recutting the threads. I was also able to take at least a quarter inch off the length of the spigot on the backplate so there wasn't so much overhang.
BTW, I've got a Kao Fong milling machine like yours, except it's a rebranded Jet.
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