After go when standing at the end of a ṛk verse, the augment n comes before the Genitive 6th-Case plural ām ,
As विद्माहि त्वा सत्पतिं शूरगोनाम्; but गवां गोत्रमुदसृजो यदङ्गिरः in the beginning of a Pada. \All rules have exceptions in the Vedas\ is an established maxim, so at the end of a Pada, sometimes this rule does not apply, as हन्तारं शत्रूणां कृधि विराजं गोपतिं गवाम् ॥,