Originally Posted by FBinNY

the photo seems to be taken close to straight on, and has little or no perspective taper. So measure the width of the 13mm (or 15mm) slot in the picture and use it to establish scale, then measure the width of the handle, and adjust for the real value. With a bit of care you should know to within a millimeter or two.

If you're lucky you'll have a fairly decisive answer, but if it's borderline, you'll have to await confirmation by an actual measurement.