I own two sets of Titleist MB irons. My 690 MB irons have the serial number on the 6 iron. However my other set (bought in 2001) has no serial numbers on any of the clubs. The set is the limited edition/production of the Tiger Woods irons (681 MB). It is the limited production set that has a "T" on the blade. Can anyone tell me why these irons do not have a serial number?