This horn has all the engraving, etc. of an S880: Elimona stamp. Yanagisawa "coin" badge. It also clearly has the S990's altissimo G key.  This is also not a "one-off."  There's another S990 at GetASax.com that's clearly stamped "990."

I'll go ahead and call this horn "Transitional," at this point.  I need to get a bunch more 880 and 990 horns from around this time to make any more of a determination.  I think it's likely that this horn has the body that became the one for the 990.  I'll also have to check other pitches.