On further reflection I'm gonna guess that the saucer was simply not glued to the secondary hull. There are hints of SOMETHING under there (look forward of the shuttlecraft top piece in the aft view polaroid) but more likely it was simple bits to keep it intact for shooting.
Not according to this pic:
There's definitely a neck there between the saucer and the secondary hull, and a substantial one. It looks to me that it's actually the Excelsior's neck, turned upside down and attached to the middle of the saucer underside. When I built my model, that's what I did, and it looks like that. However, I was hoping to have a clearer pic of that neck when I got these pics, but it was not to be.