The easiest way to identify if the Rally Pac is an OEM (vs. a repro) is to take the tach out of the camera case black housing. Here is a pic of the back of a 1965 style rally pac tach. It has a total of 3 wires (two for the tach (1 red and 1 black) and one for the light bulb (blue with a red stripe). If there are more than 2 wires for the tach is it either a repro or it has been rebuilt/modified. In addition the back of the housing will have a faint ink stamp. See attached photo.
Regards,
Ron