Ok, that makes sense looking at the diagram of the parts. As I would rotate it while pulling it, I could feel it drop into what must be a square hole. However, the square end of the bolt must not be completely going into the hole because the bolt only comes toward me a millimeter or so and never feels fully engaged.
I'll try pulling much harder with a pair of vice grips on the non-threaded part of the mounting bolt and see if I can get to seat into the square hole more completely. I wish there was some way I could put a finger (or hammer!) on the inside end of the bolt, but there doesn't appear to be a way to access it easily.
This seems like it shouldn't be this difficult. And why use a carriage bolt instead of a double-threaded bolt like the other one? Scratching my head again...
Anyway, I'll tackle this again after work and see how it goes. Thanks,
@ILENGINE!