Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 26.2.5

26.2.5

tad āhur vidūra rūpam vā etad yat trivṛc ca trayas triṃśaś ca stomau / (soma: chandomāh)

Regarding this they say: “Indeed, it is disparate in form when there are the Trivṛt and the Trayastriṃśa stomass;