Sorry, I can't help with hte serial number. By this era the Nishiki brand was under the control of Derby and I don't know who the manufacturer was and therefore don't know where the serial number was stamped. Even if we could find it, I may not be able to identify the year.

However, I do happen to have the 1992 catalog and a 1993 product list and the Backroads for those years are totally different. The 1992 was Shimano 200GS equipped and available in white or yellow, while the 1993 version was Shimano Altus equipped, though the colors aren't mentioned.