Most of the Ford service replacement gas tanks do not have drain plugs. Not sure why they sell for so much when this is a detail totally wrong.
The C5ZZ stencils would not apply at all to a factory installed gas tank. Any stencil or mark on original gas tanks would have been the USS Steel type. Original gas tanks were stamped (into the metal) with a date code, usually on the front lip/edge. The ones I have found were stamped from the bottom, so cannot be seen once installed.
The Spectra replacement tank is about the best that can be bought as a replacement.