Prepare to be disappointed... This is a reproduction 1967 - 1968 Ford Mustang clock knob and set screw (replaces Ford part # C7ZZ-15032-A) that can be cut down to work on your 67-68 standard Mercury Cougar (barely). The pictures below will guide you on this "adequate at best" solution for those of you missing this hard to find knob and set screw.
Click on images to enlarge.
The above picture is a good used original Cougar knob mounted on a 68 clock. We do not have these original Cougar knobs for sale apart from the clocks as we are missing them from about 30 clocks ourselves.
This is the same knob off the clock, note that some examples have a longer set screw than the one shown.
Here is the reproduction knob after we have cut it in half. It looks OK and works but is very cumbersome to use as it is recessed in too far.
The reason we have to cut it in half is because of where the set screw rests internally. We will not try to prove this fact, it will be obvious when you get it in hand as to why it will not work out of the box... We chose to use a Dewalt cordless grinder with a cut-off blade. A dremell tool or hacksaw work probably work as good or better.
Here is the set screw by itself. To some, this part alone will be worth the price of admission...