By coincidence I’m selling a 16610 and am reading this with interest.
20 year old watches are seldom a complete set. Normal (non wis/collectors) people just didnt care about keeping stuff like we do, and something 20 years old can conceivably gone through several sets of hands/several house moves/been inherited perhaps. There’s plenty of the proverbial “triggers brooms” around (and papers have been known to be “re manufactured” shall we say…….
I’ve put my watch up for £7100, which I think is a fair price with good provenance (although missing the original punched papers…..) but I can’t imagine a loose watch being offered for 20%/£1500 less….