The augment num is added to labh 'to get' preceded by a Preposition before the affixes khal -- īṣadduḥsuṣu kṛcchrākṛcchrārtheṣu khal [[3.3.126]], and ghañ,
Thus ईषत्प्रलम्भः, सुप्रलम्भः, दुष्प्रलम्भः, प्रलम्भः, विप्रलम्भः ॥ This is a Niyama rule, and restricts the scope of 7.1.64. Before the vowel affixes खल् and घञ्, the root लभ् gets the augment नुम् only then when it is preceded by a Preposition, and not otherwise, as ईषल्लभः, लाभो वर्तते ॥,