Primary metaphor
Samekh ס is the companion letter to the final mem ם. They both represent a kaf כ joined to a zayin ז at the top and the bottom. The covenant of marriage declared by the father (joined at the top) was fulfilled by the son (joined at the bottom). The samekh ס has rounded corners indicating the spiritual fulfillment, and the final mem ם has square corners indicating the fulfillment in the flesh.