Sutra:
51. iththAl Isvaranukku sEshamenkiRathu.

Meaning:
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.