OK Tim when I got my early 10B L.A. car all the smog was in a box, I have since installed it on the car. I looked at the bracket just now for you. Mine has a hex head captive nut. the bottom of my bracket has no holes, and at the edge of the flat part there is a slight extension of just the flat portion of about an 1/16th of an inch. Sorry no pics yet. Let me see if i can find a better image for you, of what I am talking about.
I have been looking for about 10 minitues, and I saw another survivor car with the same style as mine, but I had to really zoom in to see it. I have noticed there are MANY MANY brackets out there, all claiming to be original. Some with square, and some with hex fixed nut. Some CRG guy here I would think would have one.
. I am guessing there were multiple vendors of this bracket, so it is going to appear a little different from one to another.