Ok, I've been thinking about this all day and I can't really think of a question that I think would be challenging enough for this crew.
I am by no means a master of SOHC4 arcana. I just have good reference books.
Hinomaru set the bar pretty high and I just made a lucky/educated guess.
I don't want to hold this up any longer so I'll just throw this one out there.
What thing/feature/item was technically a single-year-only thing on the K2-K6 bikes?
Difficulty: Not carb numbers or jetting.
(And yes, I know technically the K3-K5 bike were US only. I'm really slacking here,
and I could be wrong!)
Hopefully whoever asks the next question can do better.