JM Ferat said

Thanks. Not sure... I think it's a stock 65 shaft so how would I find that? Is it the little hole on the shaft?

Separately, I found this post which has lots of helpful info including the attached pic: 65 Cuda Power Steering Box and Coupler Installation Issues

The rub is there is no "master spline notch" on my new coupler and it doesn't look like there is a master spline in the position that the pic suggests it should be in.

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