I came up with one more possibility: the overhead console was added, and the bracket is covering the original third hole (or the third hole was on the bracket that the console one replaced - sorry, I don't know as much about 67s as I do on 68s). Looking back at your photos, it seems like the two wider-spaced holes are in about the right spot for a '67 bracket. The "middle" hole looks really sloppy due to the way it hits the sheetmetal bracket, and results in an off-center hole pattern. I would try gently running a screw into that third hole to see if it's really threaded or not.
Regardless of which bracket was originally installed, your build date is very interesting in light of the info GT500KR posted.