The Cal 2500 is an excellent movement, because the Cal 1120 is an excellent movement, and the Cal 1120 is an excellent movement because the ETA 2892-A2 is an excellent movement. Basically you're talking a very common movement here, but Omega takes the top specification ETA base "Chronometer spec) which is a movement that will keep accuracy between -4 and +6 seconds per day as it is, they decorate it, and make some very minor changes as Dennis mentioned above to turn it into a Cal 1120, they then add the co-axial escapement to that to make it the Cal 2500, which is a more dramatic change, and arguably a good one, the Co-Axial escapement gives very good positional accuracy (not that its particularly noticeable on a 28,800 vhp watch anyway) and is a unique modification to Omega.
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