[VVV]

Now I don't know how to vote anymore because you have two spiritual alternatives?

But I go for the lock of the spirit which is closest to what I think is the reason.
I think it's talismanic with a lot of different symbolism.
Kind of to balance the spiritual forces of the Keris.
As Shahrial brought up the Linggam/Yoni dualism is one probable factor (on the other side of the ganja it's the pesi/ukiran = same thing).
I also find it probable that it is a kind of lock, as well as a protective shield from, the Naga, or metaphysical forces in the Keris.
But I am sure that there are several other reasons and explanations/speculations?

Michael