To be able to identify appliances for warrantee and part information, water heaters are tracked by the factory by the model number and serial number. Contact the manufacturer giving them this information and they will be able to tell the date of manufacture.
Often the first part of the serial number is the date. If it is a letter and two numbers then the letter is the month, i.e. a=Jan, b=Feb, etc. except there is no I, so j=Sept. if there are four numbers then the first two are the month and the second two are the year. American and their clones use the week of the year and then the year.