Go to triumph instructions.com and put in this part # A9638085. it will reference using a spacer that is shipped with them when new. Dunno if that's really required or not.
Dean
These look like the stock mirrors, and if they look exactly like the ones in the picture, you've already got the insert. The remaining pieces are a washer and and a bolt that fits into the bottom of the spacer. These mount onto the stock mirror locations, which are up by the levers.