The following chart details the Fender serial number schemes used from 1965 to 1976.
Serial numbers have been used in various locations on Fender instruments through the years.
So if your blackface amp has a date code stamp of OB, it was made in 1965/February.
If your tweed champ is stamped HH, it was made in 1958/August.
Up until the end of the blackface era (1967), all amps left the factory with a tube chart that had a two letter date code stamped on it.
The tube chart date code works like this: the first letter indicates the year and the second letter indicates the month.