Scary steering shaft/coupler failure!
Okay - got the coupler rebuilt and put the clip on. It's a good design.
I think what happened is that after I (or an earlier owner) killed the shear pins, the lower shaft simply rode up on its own after the coupler cover came loose. The steering column most likely didn't move backward as I was driving it.
Although not in the car yet, I did a test installation on the bench and found the following to work really well:
1) Position the clip and put one of the two legs in the hole (I used the non-recessed hole). The other leg rides on top of the coupler.
2) Start the roll pin in the recessed side
3) Drive the roll pin through gradually, massaging the first coupler leg into the pin as the pin travels through the hole
4) Work the second leg into the recessed hole and other end of the roll pin - a few light taps with a hammer and it's good and snug
5) Best part is that the clip also serves to retain the roll pin, although it probably shouldn't fall out.