It's the caliber or movement in the watch. 2500 movements were in the PO 220x.x reference watches (like the 2201.50, 2201.51, 2209.50, etc.). There were some more recent Seamasters (before the current wave ones) that had 2500 movements in them. Since a Planet Ocean is a 600m diver and most of the modern reference numbers are way longer than the old 220x reference numbers, it is just easy to refer to them by their movements.Serious question - and I think I may know the answer, but what makes a 2500 a 2500? Is it the movement? I see PO 2500, 8500, and 8900 (and possibly more). Sorry but I get a bit lost with so many different brands, model names, reference numbers, and movement calibers!
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