Sanderjo, welcome to the forums. It's extremely rare for a mass produced bicycle not to have a serial number. Nishiki were contracted to a large number of manufacturers, not just Kawamura. While the bottom bracket shell is the most common location for the serial number, it could be just anywhere on the frame, depending on the manufacturer. Other common locations include the rear dropouts, lower seat tube, upper seat tube, seat lugs and head lugs.