C above the serial number? I'd need to see pics, but my (semi-educated) guess would be 90/91. What are the numbers/letters on the drive side? You can also send mail info@gitabike.com. Ask nicely, and they may be able to give you the build year. If they do, please let me know what they say -- the more confirmed dates I get associated with serials, the easier it will be to self-date.