I cannot figure out what keeps the turn signal portion from rotating.
There is a bolt and nut that holds the two sections together and it is snug but not tight. The two sections rotate together about 1 1/2" in either direction.
I have the factory Service Manual and a Parts Manual but can't find the exploded view of the 4 speed column. Uggghh!

Well, I can't help you on your problem because I have never had a 68 column apart. But maybe that explains something for me. I was in Bellingham,WA about a month ago and pulled up in the left hand turn lane right behind a 68 Charger. He had his left turn signal on and he reached up with his RIGHT hand at the 1:00 position and turned off his turn signal! I could see it from where I was at. It was sticking straight up. So you aren't the only one who has not figured it out! good luck.

Okay, I figured it out.
The bearing housing (holds the turn signal switch) has two square headed bolts running up from the bottom. The heads ride in channels cast into the housing.
The nuts on these bolts must be just barely threaded onto the bolts to give you as much play as possible.

I borrowed this photo from an old thread so ignore the arrows. The square bolt heads are visible:


When sliding the bearing housing over the shaft and into place the square heads of the bolts side over the main column tube into the square slots on each side.

Another borrowed photo, ignore the arrow, slots are at top and bottom in this pic. The bolt heads most go outside this tube and slip into slots:






You tighten them alternating from side to side and it holds the housing firm to the main column.
Hope this helps someone else.