Originally Posted by Aardwolf

I had read about that and was fairly certain mine was ISO - it's a 1982 Holdsworth with an original Tange headset.

Fork measures 26.98
The removed crown race measures 26.8
The replacement (Tange Levin CDS) is 26.3

Rats, I'm guessing I need a JIS one.
Seems like this was known if I'd only looked: https://forum.cyclinguk.org/viewtopic.php?t=17078


Update: Tange Levin CDS also comes as JIS, but not as common.