The affixes ña (a) and an after 1. ṣaṣṭha and 2. aṣṭhama when a 'part' is meant, but not in the chandas (Vedas).,
The words भागे and अछन्दसि are understood here. By च, the affix अन् is included. Thus ष꣡ष्ठः or षा꣡ष्ठः, अ꣡ष्टमः or आ꣡ष्टमः ॥,
षष्ठ-अष्टमाभ्यां ञ च
Adhyāya 5 · Pāda 3 · Rule 50