Deciding on what constitutes a truly rare watch is difficult, IMO. It's partly to due with initial production figures and then add the decimation in numbers over time combining into current amount of watches remaining. But that is not all - a huge number of watches lies around in drawers, with their original owners (who couldn't care less about the "rarity" of their vintage watch), while some are hidden away in the collections of solitary collectors with enormous collections that never see the light of the internet, which is where a lot of us have our reference points re. rarity from. I know one of such people and his collection is
breathtaking.
Then we have the people who think that a certain aging of the features or perhaps lack of aging constitutes a rarity factor - I disagree with this.
The watches I own that are "rare" (a term I profoundly dislike the current usage of) are probably "rare" because they are not hugely collectible and therefore doesn't surface often as the demand simply isn't there. A lot of the watches I see on these last four pages of this very interesting thread falls into this category too - that doesn't detract from them at all.