I've sent several messages to Yamaha to find out the age of a recently purchase YAS-23. The employee on hand was as helpful as possible, but it seems that Yamaha lacks the pride of other instrument manufacturers and does not retain, or at least, make public a serial number reference chart.
Here's a couple of Yamaha's responses:
"Thank you. The horn which you have purchased seems to predate our current records which only go back to 1990. I wish I had more information to give. The only advice I can offer is a guess based on other horns I have seen in the past. I believe it would most likely be between 1985-1989. You could try looking for people selling them on ebay. They usually post the serial number and the age of the horn on the listing. That also may give you an idea of how old your horn is. Thank you again and good luck."