The headset sounds like it could be a JIS (?) (JSI?--I can't remember) sized headset. This was a size seen in the 80's and early 90's on some Japanese bikes. I believe the threading is the same as English, the fork is slightly larger. I had some weasel sell me a Suntour greaseguard a few years ago as an English HS--then disappear.
Regards,
John "commutes on his vintage bikes when he commutes on a bike, which is not at all enough lately" Taglia Chicago