I have read two different ways of dating a Fender amp. One site says to date by the stamped letters on the tube chart, and then another website says to read the numbers stamped on the back of the chassis. Which is correct, or are they both? My tube chart reads "QD" meaning 1967/April, and the back of the chassis reading A 14467, which means 1965... Could that possibly mean my chassis is a '65 and the cab is a '67? Thanks!