L.A.B, wrong part Mate. This is the service update showing the addition of a 12mm O ring.
What is the Triumph Service Bulletin number?
The only Triumph service bulletin I'm aware of pertaining to the flexible mount was the addition of the cup, steady rubber and distance piece, below (Service Bulletin No.306/ USA #1-70) :
Perhaps it's someone's idea of a modification they've added to the drawing (especially as it's a metric
O-ring size) below unless you have further information on the service bulletin number.