I recently read that Bhaṭṭoji Dīkṣita maintained that the numeric labels for vibhaktis have not been invented by Pāṇini, but by his predecessors:
The names of the seven vibhaktis, viz., prathamā, dvitiyā, tṛtiyā, caturthī, pañcamī, ṣaṣṭhī and saptamī, however, are not defined by Pāṇini; they are borrowed, as Bhaṭṭoji Dīkṣita says, from the predecessors in the field.The relevant passage by Bhaṭṭoji reads:
(M.D. Pandit, svatantraḥ kartā, in V.N. Jhā (ed.), Vidyāvratin, Delhi 1992, p. 133)
tatra su as jas ityādīnām saptānām trikāṇām prathamādayaḥ saptamyantāḥ prācām saṃjñās tābhir ihāpi vyavahāraḥ.If Bhaṭṭoji is right, then probably someone else before Pāṇini was already paving the way towards a non-semantic concept of the case-endings.
In this regard, the seven triplets of case-endings had the names 'first' through 'seventh' by the ancient [grammarians]. They are called like that here [i.e., in Pāṇini's grammar] as well. (BD on A 1.4.104, text from Pandit, my translation)
On the relics of different partitions of the semantic-formal reality of language in the Aṣṭādhyāyī, see Artemij Keidan's "The kāraka-vibhakti device as a heuristic tool for the compositional history of Pāṇini's Aṣṭādhyāyī" (Rivista di Studi Orientali 2011).