I got this from this link.
http://reviews.ebay.com/Buying-a-Yamaha-Ac...000000000965950
Each Yamaha guitar has a serial number stamped on the back of the neck, inside the body of the guitar. It is viewable looking through the sound hole, where the neck meets the top of the guitar. The numbers are typically eight digits. The serial number on my FG-260 twelve-string is 30710583.
Yamaha serial numbers repeat themselves every ten years. The first digit in the number is the year; therefore, mine could be from '53, '63', '73, '83, '93', or 2003. That really narrows it down, no?
Going back to the historical chart in the product brochure, I can determine that the FG-260 was produced from 1972-1973. Mine then was produced in 1973.
The next four numbers, "0710," are the date of manufacture: July 10th. The last three are the "item" number. Yamaha is pretty closed-lipped about this one; were there 583 FG-260's produced on that date or in that year? Is it number 583? Out of 583? Or, out of 10 bazzillion? To date, I haven't gotten an answer, nor have I found non-Yamaha sources to narrow it down.