I'm not sure I would trust the weight method. I have some reproduction rocker panels that are heavier than NOS rockers--FWIW.
I would look for the
date stamp on the OEM tailgate. I'm guessing the reproduction gates don't have that stamp.
Also, I think the reproduction gates are of the 70-72 style. So if you can identify
stampings for 67 or 68-69, then odds are it is an OEM gate.