The long vowel is substituted for the final of the stem before the Genitive 6th-Case plural affix nām (having the augment nuṭ).,
Karika
नामि दीर्घ आमि चेत्स्यात्कृते दीर्घे नुत् भवेत् ।
वचनाद्यत्र तन्नास्ति, नोपधायाश्च चर्म्मणाम् ॥
Thus अग्नीनाम्, वायूनाम्, कत्तॄणाम्, हर्त्तॄणाम् ॥ Theanuvritti of अण् 6.3.111 ceases. The augment नुट् 7.1.54 in नाम् is for the sake of the subsequent sutra; like 6.4.7. and the lengthening takes place after the addition of नुट् to the genitive affix आम् ॥ For if the lengthening took place before the addition of नुट्, there would be no occasion for नुट् which comes only after short stems.,