Capcom, apparently seeing the end of the
NES on the horizon, decided to pass/sell publishing rights of this game to
Nintendo of America, and therefore it is
Nintendo itself that produced and published the game. This is the only
NES Mega Man game that followed that route. Furthermore, since it was produced near the end of the
NES's lifespan, it had a limited production run and is one of the more rare
Mega Man's to find in this day and age.
As for the game itself, Capcom attempted to punch up the challenge a little bit by adding four fake robot masters to the game. Four stages - Knight Man, Yamato Man, Tomahawk Man, and Plant Man - branch off near the end of the level. One path leads to the true enemy, while another leads to a fake one. Defeating the fake one doesn't actually clear the level. Taking the true path and defeating the real robot master clears the level and earns you a piece of the fighting birdbot Beat.
Finally, on the continuity side of things this is the only game in the series so far in which Mega Man has actually captured and imprisoned Dr. Wily.