The letter ka is the substitute of the final of the word rājan , when cha is added.,
This sutra only teaches substitution: for राजन् would have taken छ by 4.2.114. Thus राजकीयम् ॥ The word देशे does not govern this sutra, not being appropriate.,
राज्ञः क च
Adhyāya 4 · Pāda 2 · Rule 140