56 is about as early as you'll find them, and I don't recall when the studs changed form 1/2" to 9/16", but that's the later cutoff point.
No reason you can't use the later wheels with 9/16" stud holes and still use the original clamp plates and nuts.
9/16 studs started with the new 1973 body style. My 70 1 ton had my 19.5 wheels so it obviously has the 1/2" studs and I saw a 72 1 ton that also has 1/2" studs.
My 19.5 wheels on the 70 did not have clamping plates. They used the same slotted flat lug nuts as my 52 18" wheels. These are hub and lug centric so I guess they do not need the plates. I also looked at the stud length on both trucks and the studs are not long enough to use a clamping plate. I have some from a 70's/80's dually and it is just will not work.
