The 990u has some toneholes that were placed differently to improve intonation. It looks like the 991 further refined the placement.
So the 990u is an updated version of the 990, but not quite the 991?thehighend said:The Greek letter 'mu' designation (i.e., what you referred to as "u" in the model #) is unknown to most people. My understanding (but I'm kind of guessing) is that Yanagisawa uses this designation when they update a model, but not sufficiently to warrant an entirely new model #.
For example, I once owned a 990u tenor, which seemed to be a model newer than a 990 but was made before the 991 model name was introduced.