Matsya Purana — The Rohiṇī–Candraśayana Vow
तदा स्नानं नरः कुर्यात् पञ्चगव्येन सर्षपैः आप्यायस्वेति तु जपेद् विद्वानष्टशतं पुनः //
tadā snānaṃ naraḥ kuryāt pañcagavyena sarṣapaiḥ āpyāyasveti tu japed vidvānaṣṭaśataṃ punaḥ //
Then one should bathe using pañcagavya (the five products of the cow) together with mustard seeds; and the learned man should again recite the mantra “āpyāyasva” eight hundred times.
This verse does not discuss pralaya; it focuses on individual ritual purification through bathing substances and prescribed mantra-japa.
It gives a practical śauca/prāyaścitta guideline: a householder (and by extension a ruler maintaining public dharma) should follow purificatory bathing and disciplined mantra repetition to restore ritual fitness.
The significance is ritual (not architectural): pañcagavya and mustard seeds are specified cleansing media, and “āpyāyasva” is to be recited 800 times as a formal japa prescription.