CPP drop spindles

I have a 1965 that I want to convert to disk breaks and later model stock calipers, disks, ball joints.

CPP has two options for the 2.5 inch drop spindles;
1963-1970
&
1971-72

What’s the actual difference between these two products?
I thought parts for 63 - 80 were pretty much interchangeable from a tire width pov.
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