I put a link to an image below. Hopefully it works for you; it did for me.
On my 68 OHC 6 exhaust manifold is...well something. It's outward appearance is that of a lever. It seems to operate a butterfly valve inside the manifold. I can freely rotate this thing clockwise from its resting 9:00 position to straight up 12:00. The shaft for this valve goes clear through the manifold and protrudes out the other side. There is nothing attached to it. I can feel it rotate with the 'lever'.
I can't find anything in the 68 manual. In the 67 manual I found a reference to a heat control valve. It had a thermal spring attached to it (on the block side). I don't have this spring.
So I kinda sorta think I know what this thing is. I read and understand what a heat control valve does. (It redirects warm exhaust air into the intake for cold startups...something like that.) If I'm anywhere close to right then mine ain't working. Fine. The engine seems to work fine. But I have an unanswered question; is the resting 9:00 position directing the exhaust into the intake or out the tail pipe? I'm baffled.
I don't know about the 6s but the original 8s had a heat control valve on the pass side exh manifold like you descibe(I can't see your link). All it was was a flapper of sorts held closed by a spring that relaxed as the manifold got hot. in the cold position it would be up and as the spring relaxed the knob on the end of the shaft would move down till it hit the stop at the full open position.
Yes, it's was a heat riser valve that was eliminated (at least in the v8s) during the '68 production year. It sent hot exhaust over engine to the left pipe until the car warmed up. Mine, like many, had the spring mechanism break and the valve would flop around at idle and sound like a loud rattle in the engine. The fix was a wire wrapped around to make it stay open.
