Like stated above, the older head's build date is located on the inside of the chassis (where all of the circuit boards are). You will have to pull the metal chassis out of the head shell. There are four screws on the bottom of the head that hold the chassis in the head shell. Remove those and then the chassis will slide out of the rear of the head shell box. I would remove the power and rectifier tubes before taking the chassis out. Thank goodness the new ones are written on the outside. Makes it so much easier. Another option is to give Mesa a call and tell them the serial #. They can probably give you a ball park date for when the head was built in their shop.