Nope, that CB isn't it. Sometimes the indentation on the stamp isn't very deep and almost gets filled with paint. Make sure the underside of the bottom bracket is very clean. Even a litle dirt, and you might not see it. You may need to get in very good light and try a couple different angles before you see it.

My '70 Professional has the serial # stamped on the Chainside edge of the bottom bracket. My '78 uperCourse has serial stamped crosswise on the bottom bracket (both on the underside). My '77 GrandPrix (not 531 tubing) has serial staped on the back of the seat tube, under the seat stays, running down the length of the tube.

If yours is 1973 or later, you should see two letters, and then a number. The number that's in that 3rd position will be the ones digit of the year - i.e. WS4XXXXX Would be 1974. From what documentation I've seen 1972 or 1973 is the watershed year for serial number change If yours is FXXXX it would be 1971. GXXXX, would, I think, be 1972.

Good luck. It's gotta be there someplace.