They might not have one, but still be genuine. I had a set of JPX EZ forged irons (2014/15 MY) and they had no serial number on them. According to Mizuno, at the time some stock/standard sets didn't have numbers but the custom sets did. As far as I know, from that time on, the serial number is merely a 6 numeric digit sequence reflecting the date when they were put together at the factory followed by a 'C' for custom build or 'S' for stock/standard supply