Yamaha does not publish serial number lists, and their serial numbers apparently are not really sequential.  I found this information on another forum:

You can find out the production date by contacting :Yamaha Band and Orchestral Division
3445 East Paris Ave. S.E.
Grand Rapids, MI 49512-0899
USA
Tel: (1) 616-940-4900
Fax: (1) 616-949-7721
Internet: www.yamaha.com/band

Ask to speak to the customer support and they can help you.

It is my understanding that you may have to wend your way through an automated answering system, but in general, they are very helpful in this regard.

Good luck!