AQ. initial [dr-], [dl-] became [r-], [l-]; [dr-|dl-] > [r-|l-]
Primitive Elvish had a number of rare initial clusters, most of which simplified in Quenya. One of these was initial dl-, dr- which reduced to l-, r-, a sound change mentioned in both the Outline of Phonetic Development [OP1] from the 1930s (PE19/37) and the Outline of Phonology [OP2] from the 1950s (PE19/79). This initial development also appeared in the Comparative Tables of phonetic development from the 1930s (PE19/21). As these initial combinations were very rare, there is only one published example of this sound change: ᴹ✶d’rāk > ᴹQ. ráka “wolf” (Ety/DARÁK). It isn’t clear what relationship this sound change had to the rule whereby initial [d] became [l] in Quenya, but I suspect the dl-, dr- combinations simplified first.