Just found this site that has an amazing set of information about how to identify your YAMAHA by VIN or ENGINE number.

Be warned that it is VERY technical and you will need a pencil and paper handy plus about half an hour and a cup of tea (or something stronger).

Just follow the link here and good luck. http://xjbikes.com/Forums/viewtopic/t=14577.html

Sooo helpful!  I was just wondering today about looking up the VIN on my project bike.. it's proven to be a mix of pieces between a 125 and 175, and is listed as a '77 but seemed to be a '76.  The PDF at the end of the posts confirmed it's a DT125FC '76 (the frame, anyhow), so that's a big help.

Also just generally interesting to know that all those numbers mean something...