Shatapatha Brahmana 4.1.5.19

4.1.5.19

tāni vā etāni | ślakṣṇāni pātrāṇi bhavanti rāsnāvamaindravāyavapātraṃ tattasya dvitīyaṃ rūpaṃ tena taddvidevatyamajakāvam maitrāvaruṇapātraṃ tattasya dvitīyaṃ rūpaṃ tena taddvidevatyamauṣṭhamāśvinapātraṃ tattasya dvitīyaṃ rūpaṃ tena taddvidevatyamatha yadaśvināviti mukhyau vā aśvināvauṣṭhamiva vā idam mukhaṃ tasmādauṣṭhamāśvinapātram bhavati

Now those vessels (other than those of the three dvidevatya grahas) are smooth. The vessel of the graha for Indra and Vâyu has a (wooden) 'belt' round it: this is its second (peculiarity of) shape, and therefore it belongs to two deities. The vessel of the graha for Mitra and Varuna is goat-like: this is its second shape, and therefore it belongs to two deities. The vessel of the graha for the Asvins is lip-shaped: this is its second shape, and therefore it belongs to two deities. And the reason why (this belongs to) the Asvins is that the Asvins are the heads (mukhya, viz. of the sacrifice), and this head (mukha) is supplied with lips: hence the vessel of the Asvina graha is lip-shaped.