S. [ðv] became [ðw]; [ðv] > [ðw]
In Sindarin, the combination ðv became ðw. The clearest description of this change in appears in notes on the development of stops + nasals from the the late 1960s:
tm, dm > ðm, later ðv, ðw (VT42/26).
This note only shows the development for v arising from the spirantalization of m, but another late note hints at similar developments for v arising from primitive b: