The affix ṭhañ always comes in the sense of matup after a stem which in composition is preceded by eka or go,
Thus एकशतमस्यास्ति = ऐकशतिकः; गौशतिकः, गौसहस्रिकः ॥
The word अतः of 5.2.115, is understood here also; so that, stems not ending in अ are not governed by this rule, such as एकविंशतिरस्यास्ति, no affixing. The word ऐकगविकः is an apparent exception, it being formed by adding ठञ् to the samasanta word एकगव 5.4.92. How do you explain गौशकटिकः formed by ठञ् from गोशकटी ? Here शकटं is taken for शकटी, both being synonyms (N. B. This opinion of the Kasika-author is not in harmony with that of Patanjali, who does not read the anuvritti of अतः into this sutra. According to him गौशकटिक is formed from गोशकटी; and एकविंशति &c, do not take this affix, because it is against usage. But the author of Kasika says—अवश्यं चात इत्यनुवर्त्यम्; द्वन्द्वापतोपगर्ह्यात् इत्येवमाद्यर्थम् -- the anuvritti of अतः must surely be read, for the sake of sutras like 5.2.128, though in the sutras, like वातातिसाराभ्यां &c there is no necessity of reading the anuvritti of अतः) ॥
The word नित्यम् 'always', is used in the sutra to indicate that मतुप् does not come in the alternative. In all the previous sutras मतुप् came in the alternative. How do you form एकद्रव्यवत् in एकद्रव्यवत्वाद् ? This is not a correct form; or it may be said to be a compound of एक and द्रव्यवत्वात् ॥,