"Mutsu beam" is a joke about how, every time you think you have something rare about to drop, it gets hit by the "Turn into Mutsu beam", and you get a Mutsu, instead. I've heard some people claim they got 12 Mutsus in a row throwing away a mountain of resources. (Bismark has equal construction time to Mutsu, as well, so you wouldn't know which you got until after it was complete.)
