I can't say for sure when it came out, but it was very recent - maybe a year before the 8500 model was released or so. The 3 level co-axial escapement was put in the 2500D, where the 2500 A, B, and C had the 2 level design. The 3 level design does not compromise on the shape of the intermediate escape wheel teeth, and as such does not suffer from the "stopping problem" that the 2 level can have...
Apparently some people on another forum have found that you can tell the 2500D in the PO by the font on the date wheel - I have never verified if this is true or not.
Cheers, Al
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