Madāk ʿarūs is a smooth stone used for grinding or rubbing perfume or similar substances for a bride. Repeated use makes the surface polished. Ṣalāyat ḥanẓal is a slab or stone used for grinding colocynth, a bitter desert fruit.
Both household objects evoke a smooth, worn, hard surface, which is the selected quality of the horse’s back.
The first half of the verse locates the two back surfaces; the second supplies their tactile resemblance. The imagery is material and concrete rather than an abstract assertion of beauty.