51. iththAl Isvaranukku sEshamenkiRathu.
Through this hidden fourth case, being enslaved to Him is being shown.
Sri PBA Swami's Sarartha Deepikai:
What is the meaning of this hidden fourth case (chaturthi vibhakti)?
It stands for bhagavat sEshatvam. That is being enslaved to Sriman Narayana who is the meaning of the akAram.