If the multivar came with the boss the spacer may be to go behind the variator nut to make up for the shorter boss?
This was in a topic i read in the last few weeks somewhere in the forum, i think someone said just throw it away, you dont need it, but i cant be sure. If you got a boss with it check its length, sure Wordslinger said it came with a 0,5mm shorter boss?
Your jet mustve been a bad one or you overtightened it when you 1st put it in?