I was told by a cue maker the Schon although Schon uses a 5/16x14 piloted joint, that it is not really that size. It is actually a few thousandths different.
I had a Scruggs, and I forget exactly, but the shaft from one would fit on the butt of the other, but not the other way... I am certain that it either has something to do with either the pilot or the thread.