I've done more digging around to figure out where the difference exists. It would appear some NC21 carbs have the smaller slides, and some have carbs which are virtually identical to the NC24. The NC21 carbs I have do not have velocity stacks, and the airbox base does not appear to have room to install the velocity stacks. Most of the NC21 carbs seem to have the velocity stacks, and browsing Japan auctions, most of the NC21 carbs look like the larger sliders. BUT, a few of them are smaller like mine.

In any case, I have confirmed that my NC24 carbs are correct for the bike, but there is still something goofed up in them. I'm working on getting a spare set of NC24 carbs and will figure out what is wrong eventually.