Found it!
My curiosity got the better of me so I dug out the Minotaur from behind a wall of rockets and the adapter was in the motor tube.
As you can see the purplish black ring is the spacer.
The adapter and spacer on the top of the picture is from the Minotaur.
I made a couple of changes from the instructions:
Glued the spacer to the top of the adapter. Makes it idiot proof. Lots of times when I was ready to launch a rocket but forgot a spacer. Moved the aft centering ring up to allow flex of the engine hook while loading and unloading the motor. (See Chris M's build thread link above for this "gotcha").
The second adapter on the bottom is from another Quest kit. Don't remember which one. But you can see that I assembled it slightly differently. The spacer is not glued on. I even labeled it so I wouldn't forget what it's for. Heh. The aft centering ring is even with the end of the motor tube but a notch is cut to allow flex of the hook. Then a retaining sleeve (BT-52) is glued further up for hook retention. These mods are not in the instructions.
Pros and cons of each:
Top method does not use any additional parts. But the end of the 24mm tube has a gap between it and the 29 mm motor tube. You may want to build it up with 1/4" masking tape to enable tape wrap motor retention. Or just use old fashioned friction fit.
Bottom method uses a sleeve not provided in the kit, and you need to cut the notch. But tape wrap motor retention is easier.
Here's pics of each adapter in place:
Hope this helps!