According to the part listed on NPD site, the change over seems to be 4/3/67 and the part is listed for 67 and 68s
Here are a few pictures of my vent cable and it indeed is broken. The plastic larger ring closer to the knob has broken away from the rest of the cable so the clip won't stay in place. It appears all you do is move the clip over the groove just behind the larger ring and push up to secure the clip in place.
Guess this is another part in need of replacing. You're right Marty - this plastic over time I'm sure gets brittle and breaks. Not a good design at all.