The ground would have been used for the formula hood (plastic). The metal hood would not need the ground wire. The drawing I posted seems to indicate that there was two harnesses used, one for steel hoods, and one for plastic hoods. The plastic hood harness has the ground wire, and the drawing shows a dimple where it would be screwed into the firewall. Also, the few other pictures of the lamp I have seen only have one screw to mount it, not two. There was a NOS one on ebay some time ago that had the one hole and it was listed for a huge amount of money. Not saying yours isn't original. The one hole mounting would be needed for the formula hood as it has a boss molded into the fiberglass that is for one screw.
My best guess is the the ground was never connected on your install because it was not needed to make the lamp work. Why drill a hole in the firewall when you do not have to.
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I built it one piece at a time and it cost me more than a dime. 
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