Only one other poster has joined us in this discussion, so either no one really knows or they haven't read this thread (or I suppose, they don't care).
So here's my assessment of it . . . if there is such a thing, the SC900 was made BEFORE the SC901 because that's how Yanagisawa usually did it. It stands to reason that an SC900 would be more similar in design to the SC901 series than the SC991.
The SC991 came to market with design changes directly from the SC901 . . . changes that were viewed by many, including Yanagisawa, as improvements over the older SC901 (namely the changes that I've already described). My memory is not THAT sharp, but I paid attention when this happened and I distinctly recall that there was no SC990 - they went from the SC901 to the SC991. Unfortunately, Yanagisawa never was known for consistency in their model designations, as pointed out in the Yanagisawa section. So ALL of this is speculation on my part.
Your initial question was what was the difference between an SC900 and an SC990. I answered that as best I could. I THINK I'm correct, but who else knows?
One other thing, while my straight Yanagisawa sopranos have the model designations stamped on them (S901 and S992), neither of my curved Yanagisawa sopranos had such stampings. I know mine is an SC902 (and my SC901) because it is of bronze (the SC901 was lacquered brass) and they were of the earlier design and that's what I specified when I bought them (and which was listed on the sales receipts). But they were not marked as such. If someone claims a curved Yanagisawa is an SC990, let them clarify it here. Otherwise, I think they made a mistake in typing out the model designations. Like I've said many times, i could be wrong. For sure, the one you are buying is of the older design. DAVE