Back in 2003 I bought a set of NEW Titleist 680 forged blades with TT S300 shafts from Golfsmith. I know that there have been MANY questions in the forum about authenticating clubs, and where to find the serial numbers. I know on newer sets that the serial numbers are on all clubs on the back side of the hosel. I guess my question is, with this particular set of clubs, is the serial number only stamped on the 6-iron as mine are? Any help with regards to this will be greatly appreciated.
Check with Cathi or Nick, but it is my understanding that if the set is non-custom, then they will only inscribe the serial number on one club, ususally the 6 iron. It sounds like this is the case with your set as the 680's came standard with S-300 shafts. I own a custom set of 690MB's and 710MB's with the serail number on all of the irons as they are a custom set.
Now, if you order a custom set (i.e. order the set with a non standard shaft shuch as the X100, or order a longer than standard shaft, custom lofts, lie, etc.) then Titleist will inscribe the serial numbers on the back of the hosel on all clubs in the set.
Based on your description, I am almost sure that your irons are authentic.
Standard set serial number on 6 iron only.
Custom set serial number on all irons.
I think having a serial number on all irons would be a good idea, it would identify all the clubs as being yours.