The "secret" answer to the question about Adam's rib holds very little value in my personal opinion. It is an irrelevant topic that neither proves or disproves biblical authenticity, unless the origin of myths and fables is a concern.
However, in order to participate, I will offer my former opinion as an active JW, and that was early man (in whatever form you may accept) had fashioned many tools and possibly weapons from the ribs of different animals.
Therefore, the author of the Genesis account could have used knowledge about that aspect of early man's daily life during the compilation of the biblical creation story.
It is a rather convenient bone to fashion into a digging tool or handle or even some type of dagger, perhaps even a musical instrument of sorts to strike some type of drum? Who knows? One thing for sure, the verses that detail the creation of woman using the rib certainly have a "primtive or prehistoric" overtone in the underlying thoughts.
That's my two-cents.