A couple of years ago someone noted that 9/16" SAE and 16mm are slightly different, and on his car, a SAE spec hub (splined 9/16") spun on his metric steering shaft splines, after tightening the nut. The splines are of course 50 yrs old and possibly slightly worn, making for even less engagement. Modified Panteras, especially with big tires will often get more than stock force applied to the steering shaft. NOT a good situation to have your steering parts rattle around before tightening! I worried enough about this for my wife's driving open tracks that I made my own hub by chopping up a junk (stock) steering wheel and adapting the stock splined center.
As I'd mentioned when this last came up, with such a spline mismatch you can temporarily force a piece of thin brass shim stock inside the hub splines for track work, but I'd be holding my breath in each turn street driving in our mountains.... And I've had a full life!