Henry and all,
I did some further checks on the Nikon zoom. The MC II also shows pincushion at all focal lengths. I cannot say if there is more or less than in the MC. I would not consider it excessive, but it was more obvious than the very mild barrel distortion in another zoom eyepiece I was comparing it to (not one of those in your test).
Another thing I ended up testing was how well the eyepiece holds its focus throughout the zoom range. Here, although I knew from experience that the MC II does very well, I was nevertheless surprised to see the result. When focused to best focus at either extreme (7mm or 21mm) with an artificial star and a booster, I could go through the entire zoom range without getting any additional rings to the diffraction pattern. Meanwhile, the other zoom when optimally focused at the shortest focal length would show four rings at the longest focal lengt. I would thus consider the Nikon MC II a true zoom, not an altogether common achievement.