According to the Comparative Tables, this diphthong only appeared in the East Danian, where it was preserved from the primitive diphthong ✶[au], and also developed from primitive ✶[ou] when short [ŏ] became [a] in Primitive Danian, whereas [au] became [ō] in Ossiriandic from both these sources (PE19/25). In The Etymologies, however, laur “gold” was explicitly marked Ossiriandic (Ety/LÁWAR), so perhaps Tolkien changed his mind about the development of this diphthong.