The affix kha (īna) and khañ comes in the sense of 'a mirror' after the words 1. yathāmukha and 2. saṃmukha being in Genitive 6th-Case in construction.,
The word दर्शनः means that in which something is seen, such as, looking glass, mirror &c, which reflect things. The word यथामुख is an Avyayibhava compound, the meaning of यथा being that of likeness 2.1.6. Thus यथामुखं दर्शनः = यथामुखीनः 'showing or reflecting the likeness of the face' — a mirror. सर्वस्य मुखस्य दर्शनः = सम्मुखीनः 'showing or reflecting the whole face' - a looking-glass.,