An affix comes after a word in the Locative -7th case in construction in the sense of 'who stays there'.,
The anuvritti of कालात् ceases. The sense of भव here is 'existence', and not that of 'birth' : because the sense of 'birth' is taught in sutra तत्र जातः 4.3.25. Thus, स्रुघ्ने भवः = स्रौघ्नः \who stays in Srughna\. माथुरः, राष्ट्रियः ॥
The repetition of तत्र in the sutra though its annuvritti could have been drawn from the previous sutras, is for the sake of stopping the annuvritti of तदस्य ॥,