This Ilkorin phonetic rule is similar to sound changes that occurred in later Old Sindarin but not in contemporaneous Old Noldorin. Afterwards, the initial [s] was lost to leave only the voiceless initial consonants, as indicated on the Comparative Tables (PE19/20). There are no attested examples of this development in The Etymologies.

Conceptual Development: In later linguistic notes from around 1950, the “Beleriandic” development seems to have been initial [s-] to [θ-] instead: “sr > thr (þr)” (PE18/94), whereas simple initial y, w, l, r always became unvoiced (PE18/105). Both these developments were similar to the rules of Old Noldorin and Noldorin, so perhaps Tolkien switched the phonetic rules between the two languages; the later Sindarin language had the same development as the Ilkorin of the 1930s. This revision occurred shortly before Tolkien abandoned the Ilkorin language entirely.