The affix aṇ -- prāgdīvyato'ṇ [[4.1.83]] comes after a word in construction in the Instrumental -3rd case, in the sense of seen - the thing seen by the one whose name is in the Instrumental -3rd case, being the sāma veda,
Thus क्रुञ्चेन दृष्टं = क्रौञ्चं साम \the portion of the Sama seen by (or revealed to) Kruncha.\ So also वासिष्ठम्, वैश्वामित्रम् ॥,