Depending upon the year the serial number will be stamped on either the outside or inside of the left rear dropout. The first four numbers indicated the month and year of manufacture. If you have a frame dated September of later it's actually the next model year frame. The top picture below is a 16" frame produced in January of 1999 and is stamped on the outside of the left rear dropout. The bottom picture is of a 14.5" frame produced in September of 1992 but it's actually a 1993 model year frame. Hope this helps