Hello Art & All,
I've been working with a D1-3 camlock lathe (Harrison M250) for nearly 40 years and have perhaps half a dozen spindle mounted "things", two of which I made my own D1-3 backplates for, and none of the cam locks engage at the same point in the cam rotation. That's the nature of the beast, despite how carefully you adjust the studs. Some studs engage immediately at the beginning of the rotation, some in the middle, and some engage so late you think it might rotate through. Certainly fiddle a bit with the stud to get the best setting, but if it engages at all, and doesn't rotate through, you'll be fine.
As an aside, when I made my back plates I bought ready-made studs (Polish Bison spares IIRC) which were surprisingly cheap at the time, and used a raw disc of cast iron for the plate. I found a drawing of the D1-3 plate geometry online somewhere and downloaded. At first I was intimidated by the prospect of machining the internal taper, and to some extent the 3-stud pattern, but with a little Hi-spot blue, and repeated try-n-fits, the taper fit turned out beautifully.
