Could be either 26.4 or 27.0 depending on the intended market. 27.0 or JIS (japan industrial standard) if intended for the Japanese market, 26.4 (ISO) if intended for export to the USA or Europe.
You need to also need to be concerned about your frame. JIS headsets will have 30mm cups, ISO will be 30.2. You'll notice that both the frame and fork can be changed from JIS to ISO but not the other way around. If either the frame or fork is ISO, buy an ISO headset, and have the other component converted.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.