Is there any consistency (industry standard) in tenon tapers between different manufacturers? In particular, are the tenons machined with Morse tapers, or perhaps another standardized taper? I'm not asking about locknut threading, but just about consistency in the taper angle.

The ones I have measured are close to .040 per inch, which is not a Morse taper. Machine tapers are defined by both the amount of taper and specific sizes. Tenons are pretty much all different and don't really line up with any specific size.

No. There's isn't even consistency inside individual manufacturers (e.g. large Holtons seem to be at the Brown & Sharpe .042/1" rate, in #6 and #7 territory; small are Morse).

But there are reasons.

Bachs are all essentially Morse rate (~.050/1") and fall within the Morse #2 range, though would need to be approached with a Jarno #5. Thick walls for the bore size, 1.25" tenon, 1" insertion (at least to begin with). Sturdy tenon/significant room for wear/sizeable bore gap; not sure which was the main priority, but I'm sure all were factors.

Small Kings are steeper than Morse, around the .0625/1" NPT rate. Combined with the slide tube being recessed halfway up the tenon, which goes on to reach a crunchably-thin-wall final diameter, you get the smallest bore gap possible without going over to a Rath-style taperless system. Again, a meaningful choice.

Of course, there's even more standing in the way of interchangeability; if the taper doesn't getcha, the slide length or lock threads will.