Hello again,
I just compared the "NF5 L" to "KV3 L" shift fork (picture KV3L available on CMSNL), they are different, so that could be the problem, see the attached picture.
As I have tried, the NF5 fork clamps sometimes when I shift up in 5th gear, sometimes it works, sometimes not, I tkink that must be the problem, cant imagine anything else.

Both of the shafts, the drum and the star selector look good, dont want to take apart the shafts with the different gears, now plan is to search and buy a "KV3 L" shift fork and the washer for under the clutch basket nut and then reassemble alle parts.

There hasn't to be a o ring on a wet clutch main shaft, correct?
Can you look at the picture and confirm it looks like a NSR (and not RS250 NF5) gearbox?

What do you think? Still cant believe someone put in a shift fork of another bike.

Kind regards, Felix