Just picked it up and played a bit. First of all, Brian's a gentleman and a great shooter. Thanks, Brian!
Unfortunately, I didn't really have a chance to give it a fair shake. I had a back spasm this morning and was having serious trouble just moving around...but I played, albeit like crap, for a couple of hours anyway. The cue hits just like my old D19. Nice and solid. The joint is very straight and square, and all of my old McDermott shafts happily swap with no problem, just like it should. I ended up playing most of the time with my old shaft. The tip on the new one is very old and kind of crumbly when I reshaped it to my preference, so it wasn't really going to play to it's potential. Pretty much that's exactly what I expected on a cue that's this old and unplayed, but I tried it out anyway. It felt nice and has a nice taper. I have no reason to believe it won't hit like all my other McD shafts when I install the new tip.
With my old shaft (my current player) the cue plays and hits very well indeed. I'm very happy. Pretty much feels just like an old McDermott should, and beyond that not much should be asked of an old McDermott.
And now for the interesting stuff!
This must have been some sort of transitional cue. The bumper is the new style bumper, and it's a different material than their current ones. This one was like rubber with a hard plastic outer material. The current bumper is much softer. They're almost exactly interchangeable, but the new one is just SLIGHTLY bigger and needs to be crammed in there, but it's a nice, tight fit!

The little retaining lip even lines up perfectly. The old bumper was stuck in there pretty good, and eventually cracked and crumbled apart when I tried to remove it. Ha ha....no problem. I was going to replace it anyway with the new, softer one because the old one makes a "clink" noise when you put it on the floor.
The other odd thing is the weight bolt. It's neither the old style nor the new style. A 1/4" hex wrench fits the head perfectly, but there's something off about the thread. I need to look more closely at it. It looks a lot like the new style weight bolt, but I happen to have some here and they don't fit. It's not just that it won't thread in. The hole is simply too small. Maybe these are some shop made prototypes and they used some sort of proprietary thread? Who knows.
Given that it's a new style bumper and sorta new style weight bolt, which I think were introduced around 1993 or so, I'm thinking that Jim wasn't cleaning up his shop...he was maybe cleaning OUT his shop in preparation for selling the company to his sons. It looks like McDermott may have been trying out their new weight bolt/bumper setup, and this cue perhaps got prototype or pre-production hardware.
I'm going to have my local guy do whatever he needs to do in order for their new weight bolts to fit. The new ones look slightly larger than the old ones, and it doesn't look like the bore is threaded very far down, so it may be as simple as boring it to the correct diameter and threading it. Should be a very quick fix. I thought about keeping it original, but I like a heavier cue (just under 20oz) and this will be my new daily player so I want to get it right.
The points on the cue are beautiful. Sharp, straight...NICE. Even nicer than my D19 points. Whoever made the blanks did it right. I'd rather have no points than those stupid, rounded, CNC inlay points. These are splice style points, just like they used to be up to and including the D series.
Maybe I'll give Bill at Cornerstone a call tomorrow and see if he has anymore information. I'd like to know more about exactly what it is I have here because it's definitely unique!
Thanks again, Brian! It was a pleasure, I love the cue, and thanks for playing a bit. I hope we can do it again soon when my back is straightened out.

As soon as I learn more about it, I'll let you know.