I think we are talking the same stuff here, but I took the liberty of annotating the shop manual page you linked to so we can be clear.
If you look here you will see that I've marked two things. The item marked "A" is what I am calling the octane selector. You can loosen this to do minor timing adjustments, but it is not meant to be used to set the initial timing. The initial timing should be set with the octane selector in the middle ("0" position) so that you would have some room to make adjustments later.
The item marked "B" is the nut-side of what I am calling the distributor clamp. I could see also calling it a "vacuum advance clamp" since it really does secure the distributor body to the vacuum advance bracket (which is secured to the block via the octane selector bolt). The other side of "B" is a regular flat-head screw that I am having trouble getting good access to.
The shop manual says (highlighted in green) to "loosen the distributor clamp and rotate the distributor body...". That is why I'm surprised that the screw is such a pain to access, especially if it is part of setting the timing. Perhaps the engineers figured that once the timing was set at the factory it would be "close enough" such that the octane selector would be the place where owners or repair garages would adjust the timing. That would be consistent with what you said about it being an "assembly point" and not an "adjustment point". What do you think?
Sorry for the long post--I just like to be as clear as possible. And thanks entertaining my questions. This is my first time working on a vehicle like this, and I'm the kind of guy that "needs the data".
