New Josquin Edition (NJE) 26.10
According to Grove Music Online, "Puer natus est nobis Haec dies" is "attrib. Josquin in D-Dl Grimma 51; attrib. in 1538 to Mouton (Sup, A, T), Mahu (B); probably by Mahu (? Mouton attrib. due to confusion with his better-attested Puer natus est Gloria)".