I got a call from Kirk Evans decades ago concerning loose steering wheel adapters. Two Pantera owners had contacted KIrk, having found that 5/8" splines (US measurement= 0.06250") are almost but not quite equivalent to 16mm (Euro measurement= 0. 6299") with the same 36-spline count. Things go together but the joint feels loose until you tighten the nut. I wrote up our discussion in the POCA Newsletter at the time. So it's in the Archives and may also be here in PI's comm-archives- with which I'm not familiar.
The size difference is about 1/2 the depth of 36-spline engagement depth! So if a U.S made adapter or u-joint is on the low side of tolerance, or the Pantera steering shaft splines are a bit worn (or both), in an emergency at 150 mph or so, a driver can find the steering wheel is no longer tightly connected to the front tires. There should be ZERO slop in the fit of these steering parts! BTW, thats why I don't use the once-popular quick-disconnects- they always felt loose and worrisome to me.
I believe thats also the reason for the taper fit section on the hubs: with it you're not relying just on the splines for a tight, firm connection. When I made my own steering wheel adapter for a larger OD MOMO wheel (to give my little Judy more wheel leverage), I cut apart an old stock steering wheel and used the steel splined center for my adapter to the shaft. No scary things have happened yet in over 30 years of fast mountain touring, open-track or autocrossing. YMMV.