Dear Vstibbard, thanks a lot for your answer.
Indeed, i did?nt fit all the rear damper parts back to understand the point, i should have seen what you mean immediatly.
I read all the gas litteature on the forum and understood that :
The collar in the photo is in fact a spacer as you mentionned.
I do need a shouldered specific top nut or a top hat sleeve under the nut to center the shaft on the lotocone.
As i do not have lathe, i will ask to lotus parts dealer, but i?m afraid they don?t sell this part for gas shocks
As exemple, keveldon propose this nut for red and yellow koni, spax, avo, but not for gas.
I?m really desappointed the bolt on kit i bought don?t include these unavoidable centering parts... neither a usefull install manual
Anyway, thanks again to take your time to light up my way