Great question, I am sure Jeff is going to chime in, but I will give my take. I believe that the Thermactor engines of 66, 6 cylinders, did not have the clamp. My air cleaner, which I know is original to my 66 6 cylinder thermactor CA build (6J is the air cleaner code on mine, consistent with my 18K potential build date), there are two extra hoses going to it. One on the passenger side, which routes around the back of the valve cover and to the anti back fire valve. A second one, from the valve cover oil cap, to the air cleaner snorkel.
That connection is why I believe the thermactor 66 , 6 cylinder cars do not have the clip, the routing of the heater hose would not work in that setup, mine just lays across the valve cover and then through the back of the choke cap, which from reading what I have found, Osborne manuals, etc, seems to be right.
I have attached a couple of pics, the thermactor does include a lot of stuff that normally isn't in the way on any other non Thermactor car.
I don't believe that there would be any other reason to not have that clip on the valve cover.
BTW, great looking engine bay you have there!