So it could be 5/8" or it could be 50 mm. That's what you're saying isn't it (with the measurement error and all?)
EDIT: Hate to play Devil's Advacate but you can't tell by looking if it's metric or SAE can you?
No, what I'm saying is that a 5/8" diameter works out to a circumference of 1.9634954 (plus a bit) inches. Converting that to metric results in a circumference of 49.87278mm. The posters who have measured their clamp shaft circumferences have measured 50mm, and I don't think their measurement methods are accurate enough to believe that the difference of 0.12722mm (0.005 inches plus a bit) is large enough to think that the shaft is anything other than a 5/8" shaft.
If I worked it backwards from a circumference of 50mm, the metric diameter would be 15.9154943mm plus a bit, and I have a sneaking suspicion that no manufacturer deliberately makes metric sized shafts in that diameter. If we were to assume a 16mm diameter, then the circumference would be 50.2655mm (roughly).
So, if they were to purchase clamps with a shaft diameter of 5/8", and the original was actually 16mm, the 5/8" shaft would fit, as that is 15.875mm, leaving a slop of 0.125mm (roughly 0.005 inches).
The real answer is to find someone with a micrometer.