The half-shell, the position and color of the woman's hair, and her nudity are all reminiscent of Botticelli's very famous painting The Birth of Venus:
While seeing the influence of Botticelli is interesting in and of itself, it is also fascinating to look a the changes that artist Boris Vellejo made to the original image--most notably, in this case, the position of the woman's arms. While I haven't read this Heinlein novel, a brief glance at the Wikipedia description of the novel makes me think that this change reflects the more assertive, triumphant claiming of sexuality by Maureen Johnson Smith Long, the novel's protagonist.
Another great example of this is Tristan Elwell's cover for Jo Walton's novel Tooth and Claw:
...which evokes perhaps the most famous painting of them all, da Vinci's Mona Lisa:
Overall, Gallo's article is a fascinating one, and people in the comments have added other connections that they have noticed as well. I'd definitely recommend taking a look for yourself, and if you think of other connections, let us know!