Not at all sure I agree with your interpretation of the term "AERODINAMICA", and thus the count of USA Quad's in any particular model year. My understanding is that all series 3's (worldwide) are covered by that term, not just the Quad's, but of course, I could be wrong. We didn't use the term here in the states, but most of the other markets did.According to Cris Rees', "ORIGINAL ALFA ROMEO SPIDER" in 89 there were 1394 USA Series 3 AERODINAMICA Spider 2000 models produced. Aerodinamica is the skirted Quad version.
World Upholstery literature States that from 85 to 89 there were 2594 USA Quads made.
The seller is a bit optimistic on the rarity of the 89 Quad, methinks.
Does Rees offer any count of all USA S3 Spiders of a particular year, inclusive or exclusive of the Quad's? That would help to clear things up.