When I read the title, I thought you were referring to the ciphering machine of the Nazi's, which by its conception was brilliant since it was a completely reversible ciphering, completely analogic and not easily breakable. Then I laughed my self out because, and I am sure this was not the intension as I didn't bother looking at it yet, it reminded me of a poem adolescents (at least of my era) say between them in order to laugh.
(SPOILER) If you are easily offended please skip this message
This is the poem:
Greek: Έρως στα μάτια φαίνεται, στο στόμα κατεβαίνει, και αν κατεβεί πιο χαμηλά, σε εννέα μήνες βγαίνει...
Greeklish: Eros sta matia faenehteh, sto stoma kateveni, keh an katehvi pio chamila, se ennea mines vghaeni.
and
English: Love shows up in the eyes, it gets lower to lips, and if it gets even lower, in nine months it gets out...




(Eros = Erotas = the Ancient Greek God of love that shooted his arrows in the hearts of people and love has beginning)
So in order to complete the not intentional message, just put a head in the missing rectangle because it contains both the eyes and the lips.

