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@gbj_tester
I already did that, some years ago (when rec.bicycles.tech was the centre for such discussions)
Here's the answer, got by measuring Campag and Shimano axles:
JIS taper = 2°
ISO taper = 2°
JIS square at end of taper = 12.7mm
ISO square at end of taper = 12.7mm
ISO axle has a 1mm x 45° chamfer beyond the end of the 2° taper. This results in an overall length 2mm greater than a JIS axle of the same fit. In extreme circumstances, your crank bolts can bottom out on this extension if the crank was designed without taking it into account. In general, distance between the cranks will be 2mm less if you use an ISO BB compared with a JIS BB of the same nominal length. Assuming a symmetrical BB, this means a chainline moved inboard 1mm if you replace a 110mm JIS BB with a 110mm ISO one.