I've always thought the FONs were an excellent way to date a guitar. I recently had a customer bring in his fifties ES-125. I looked at the FON and it started with a "Y", said "It's a 1953". He said that in fact it was a 1952 as he got the guitar as a gift from his parents when he turned 13. He is very sure of the date as remembers exactly when he got it. I will ask him additional questions when he picks it back up, just needs the tuners replaced as the old buttons have shrunk and are falling apart.