changing brake servo and master cylinder Disco 3

Hi folks
Bought a new servo, oil in servo main dealer can't take the car in until next month with the holidays coming I
need toget this done. In the box with the new servo was a small bag with a 0 ring and two small spacers, with one side concave and the other convex my think is that one fits up the inside off the master cylinder so the rod from the servo pushes on that.Am i right in think this any idea folks,
Thanks.