I have sevderal tanks. There is generally a part number on the steel bracket on the back side. I ahve never seen adate code. Only a part number. There are a few part numbers.
I have seen a tank with a part number tag soldered to the top side. It was ugly.
The date code can be found on the water neck where the flange attaches to the intake, above one of the bolt holes. It will be in the ..."2d" date code format... (April 1962)ETC...