Sounds like you've already read the section on this site on sheet metal date code stamping. They can be hard to spot, sometimes the repaint is very thick, or the stamp may have been struck light or half way off of the corner, etc. FWIW, it may not be as apparent as the pictures in the CRG write-up and may require removal of the paint. Use a stripper or paint thinner if you have to remove the paint, not a wire brush or abrasive blaster.