Doug:
Here is what I think I know.
The 2H dial is the dial the B1 base was designed for.
The D base had to take a #4 dial, and could not take a #2, by virtue of the finger stop being mounted on the outer edge of the #2, and the D mount having the recessed dial mount, and apparently being designed just of the #4 dial.
I am sure many, many B mounts had field replacement dials that were #4 dials, and I have no idea whether later B mounts had #4 dials as a stock dial or not. The B mounts were not in production very long, and were quicky replaced by the D mount due to the B base being too narrow for dialing without holding onto the phone.
Many original #4 dials were converted from refurbished #2 dials.
I am responding from work, so I don't have Larry Wolff's book available, but he may have more information in the book. I know he dedicates a lot of space to the dial evolution and the B1/D1 base evolution.