For reference, I have found a site giving yamaha year codes. I have posted a picture of a serial number, look for the U and the year on that one is 1978.
My 2005 F115 has a plastic sticker with "Mfg Date" or something similar. I don't recall the year designation being embedded in the model number. What I'm trying to say (poorly, I know) is that I believe they changed that mid-run in the 2005 production year.