Hello Mme Coultas,

You are right — this part needs to be fitted manually.
The mounting follows the same order as shown in the photo on the website.

You should attach the left-hand metal piece to the bass strap using the two rivets provided, inserting the screw part that will lock into the roller.
Then, place the small mounting plate on the left-hand side of the accordion body, make a small recess in the wood to allow the roller to move freely, and finally install the roller into the plate.

Best regards,
Jean Malle