Thanks guys - Yea that bit is gonna suck a bit but that is learning for you. I wont forget it ever again :)
I think one of the o-rings go in the chain adjuster, not sure why it needs it.
https://www.diyspareparts.com/parts/stihl/diagrams/ms660-magnum/f9e404cc-012c-4715-8264/
number 14...