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Could be they were selling them even cheaper than normal because in 1989 they did the first major revision of the model in 37 years (new reference number and caliber). Unthinkable nowadays that a line could exist pretty much unchanged for that length of time.
@Olly398 if you get a chance can you measure the case width please, mine is 35mm which again is peculiar as Air-Kings are usually 34. The date complication makes them a bit thicker too I believe.
Maybe they just pushed advertising hard on them between those dates? Might have been as simple as that I guess. Or just that they suddenly became fashionable.
At the time the 5500 and 5700 were the cheapest Rolexes I think.