Yes, the SSK is rare. With their PCs, IBM offered either the full size or SSK. Since they were the same price, most buyers opted for the full size model M. Additionally, when Unicomp bought out Lexmark's buckling spring manufacturing equipment, somehow they didn't get the molds to make SSK cases and barrel plates.
So the only existing SSKs are those manufactured between 1987 and 1995, or thereabouts.