The affix ṇini comes in the sense of 'enounced by him' after a word in the Instrumental -3rd case in construction when it denotes a brāhmaṇa or a kalpa-work enounced by ancient sages.,
This sutra further gives the sense of the affix. पुराण means 'Ancient Sages.' equivalent to पुरातण. Of Brahmana literature, the examples are, शाट्यायनिनः, एतेरायणः ॥ Kalpa work : as पैङ्गीकल्पः, आरुणपराजी ॥
Why do we say पुराणप्रोक्तेषु 'announced by ancient sages.'? Observe याज्ञवल्कानि ब्राह्मणानि, आश्मरथः कल्पः ॥ 6.4.151 Yajnavalkya &c being sages of modern times. The तद् विषय affix 4.2.66 will not also apply to these, because the word ब्राह्मण in 4.2.66 is restricted to पुराणप्रोक्त of the present sutra.,