Hi Brian,
You'll have to partially disassemble the Rally Pac to determine it's originality. One thing of note is the that Rally Pac appears to be a period correct over-the-counter accessory (since the script in the tach reads "Ford" as opposed to Mustang in a factory Rally Pac.
Unscrew the 3 large black Phillips screws at the back of the housing to release the tach and the clock from the housing. The original tach's are made by Faira and a have a stamping on the back of the pod that sit inside the black Rally Pac housing. They are also stamped 8 CYL or 6 CYL based on the application they were designed for. They are also a 3 wire design (red and black for the tach) and a blue with red stripe for the light. The light used is a #1895 globe shaped bulb.
In order to see if the clock is original you need to disassemble it even further and remove the screws that hold the lens and lens cover in place. If it has been modified (to a quartz movement) then it may have been original but the movement was changed. If it's original you will see the mechanical movement of the clock. It is my understand that Ford used two different clock mechanisms in the Rally Pac. Mine is a Borg clock, denoted by a large B stamped on the back of the housing. Hope this helps.
Regards,
Ron
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