"Question: The back seat of a convertible provides a narrower area for passengers than that of a roofed car. Anyone know for certain the number of belts provided back there by the factory?"

I am not positive from remembering the cars (more time in the drivers seat ;-)) but the parts book lists two only, and the center or third belt only for A/B/C bodies, however, there are changes as to the body styles in the book (before or during print, not by a parts guy). In my mind that eludes to the possiblity of some early cars having them, or issues with the vert like you say. The problem I always have with the terms "always" or "never" is that with Chrysler, it was never that.