The affix iṣṇuc in the sense of 'the agent having such a habit etc.' comes in the chandas (Vedas) after the verb that ends in the causative ṇi,
The Causatives take this affix in the Vedic literature. As दृषदं धारयिष्णवः 'who hold the stones'; वीरुधः पारयिष्णवः ।,