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The little plastic spacers for the back bolts are 3mm thick and 19mm diameter (measured an NOS one). If you want to see one. ..... type "Yamaha 90560-06151" on an EBay search. There are a few available. Good luck.
I've already had a look on ebay. The problem is, I'm in the UK, and so the postage costs are rather prohibitive! However, I've got a good supply of bits and bobs that I can look through to find something suitable.