- Rare models often had a limited production run because they were not popular, and sometimes for good reason.
- Similarly, it is not generally possible to apply the "all other factors being equal" criterion, since by definition different models have different designs/functions, which affects their desirability.
- A distinction that is too fine (e.g. between "Seamaster" and "Seamaster Geneve") will not register on the market. They will just be seen as one of many variants (of Seamaster in this case), and not a separate "rare" model.
- Marketing and emotional attachment greatly trumps rarity as a determinant of desirability. The popularity of "Seamaster" models would be an example of this. But the desirability of Rolex is obviously the most dramatic case study.
There are many other caveats. Value is a function of rarity and desirability, and the latter is not simple to determine; it requires actual knowledge of details, not just a simple calculation of production numbers. Basically, I think the main message being sent to the OP is that it is not possible to skip the part of watch valuation where you actually become an expert by looking at hundreds and hundreds of auctions/sales and develop a detailed understanding of why those watches brought the prices that they did.