For the final affixes mat and vas there is substituted ru in the Vocative Case singular, in the chandas (Vedas).,
The word संहितायाम् is understood here. A word ending in the affix मत् or वस्, changes its final त् or स् into र्, in the Chhandas, in the Vocative singular. Thus मत् :- इन्द्र॑ मरुत्व इह पा॑हि सोम॒॒म् (Rig. III. 51. 7) हरिवो मेदिनं त्वा ॥ Here मरुत्वः is Vocative Singular of मरुत्वत् and हरिवः of हरिवत्, meaning \he who is possessed of Maruts or Hari horses\. The म् of मत् is changed to व by 8.2.103; 8.2.15. The base मरुत् before this termination is Bha by 1.4.19. The affix सु (nominative singular) is elided by (VI. I. 68); the त् is elided by 8.2.23, and the न् (of नुम् (VII. I. 70)) is changed to र् by this sutra. So also इन्द्रायाहि तूतुजानः उपब्रह्माणि हरिवः (Rig. I. 3. 6).
With वस् :- मीढ्व॑स्तो॒॒काय॒॒ तन॑याय मृळ (Rig. II. 33. 14), इन्द्रसाह्वः ॥ See (VI. I. 12), for the formation of मीढ्वान्, साह्वान् with क्वसु ॥
Why do we say 'of मत् and वस्'? Observe ब्रह्मन् स्तोष्यामः ॥ Why do we say in the Vocative Singular? Observe य एवं विद्वानग्निमादत्ते ॥ Why do we say in the Chhandas? Observe हे गोमन्, हे पपिवन् ॥
Vart:- The affix वन् should also be enumerated. The affixes क्वनिप् and वनिप् are both meant here. Its final is also changed to रु in the Vocative Singular. As यस्त्वायन्तं वसुना प्रातरित्वः ॥ The word प्रातरित्वन् is formed by adding क्वनिप् to the root इ (इण्) preceded by प्रातः ॥ See 3.2.75. The तुक् is added by (VI. I. 71).
Vart:- The finals of भवत्, भगवत् and अघवत् are changed to रु, optionally in the secular as well as the sacred literature, and the syllable अव् of these words is changed to ओ ॥ This is a general rule, applying both in the Vedas and the Bhasha. Thus भवत् — हे भोः or हे भवन् ॥ भगवत् — हे भगोः or हे भगवन् ॥ अघवत् — हे अघोः or हे अघवन् ॥ Or these words भोः भगोः and अघोः are so irregularly formed by the acharya himself in 7.3.17. These are found in other numbers than the Vocative Singular, as भो देवदत्तयज्ञदत्तौ, भो देवदत्तयज्ञदत्तविष्णुमित्राः ॥ It is found in connection with Feminine nouns also, as भो ब्राह्मणि ॥
The adhikara of the word संहितायां does not exert any apparent influence in this sutra. It however has influence in those subsequent sutras, where the change in one word is occasioned by another word on account of sanhita or juxta-position, as in 8.3.7.
See 8.2.3 (Vartika 2) by which the lopa is considered siddha in हरिवः, otherwise there would have been no उ change in हरिवो मे &c.,