I have a couple of Rudelli headsets on a of bicycles { Coppi 1997, Moser Forma/96 } that use Italian-threading and standard forks.
But the good news, the handcrafted NivaCrMo fork and the headset that I want to get for my new titatanium bicycle, are both "ISO standard", with a declared "crown travel 26.4mm".

Good!!!

Just, to makes thinks more complicated, if the headset manufactor's website says "26.4mm", and you you have to use your immagination to guess to what that number is referring to.
"crown travel", you guess, but when you want to doubple check in the reseller's description ... well you found nothing but "ECxxx".

What is ECxxx?!?

The Customer Service explained that the crown race are the bearings at the bottom next to the fork

EC26.4 -> External Cup, crown travel=26.4mm

so, "ECxxx" refers to External Cup (vintage headset), while the xxx number refers to the inside diameter of the headtube.
However these are not actual measurements they are a way of classifying headset groups.