The thaṭ is the augment of ḍaṭ in the chandas (Vedas) after a Numeral ending in n and not preceded by another Numeral as well as the augment maṭ,
Thus पंचथः, सप्तथः or पंचमः, सप्तमः ॥ As पर्णमयानि पञ्चथानि भवन्ति and पञ्चममिन्द्रियमस्यापाञ्च्यमन् ॥ See 5.2.56.,