from my experience, it works one way but not the other. I have an old mcdermott cue (3/8x10) and a new OB cue (modified 3/8x10). The shaft for the OB would screw onto my mcdermott butt no problem. The shaft for my mcdermott would NOT screw onto my OB's butt. It would go a few turns and then "bottom out" and get very tight, obviously not fitting. Maybe it could be forced, but who is going to do that to their cue? I had a local cuemaker re-tap my mcdermotts shaft with modified threads so it can be used on both cues.
It depends on who made the shaft. There are a lot of people who think the standard hole for all 3/8x10's is 5/16".
It is not. But they do it because someone said it was.