Wolfgang Weiss
Marin Fan
I don't have caliber so I cannot measure it.Measure the crown race. ......easiest way.
Iso is standard size....26.4mm
Jis is 27.0mm
Crib sheet here
Sheldon Brown's Bicycle Headset Dimension Crib Sheet
Useful headset (steering bearing) dimensions and conversions for the cyclist/mechanicwww.sheldonbrown.com
Unless its old old old french, its unlikely to be french size..