[Note: This sound change probably took place before the previous one (Part 32) but I didn't notice things were out of order and I don't want to alter the previous post. I will fix the ordering in Eldamo.]
OS. syllabic initial [s] became [es]; [ṣ-] > [es-]
In some cases in Sindarin, an initial [s] before a consonant could become syllabic and develop into [es-], such as ✶spini- > S. esgal “cast shadow” (PE17/184), ✶spini- > S. esbin “thin thread” (PE17/17), or √STEN > S. esten “short” (PE17/184). All of the attested examples of this development involve an initial [s] followed by a voiceless stop.
Conceptual Development: In the Gnomish and Early Noldorin of the 1910s and 1920s, it seems syllabic [ṣ] was part of the normal phonetic inventory of Primitive Elvish, although it was rare. In Gnomish it seems that [ṣ] became [us], at least before labialized consonants: ᴱ√ṢQṢ > G. usc or usg “fog” vs. ᴱQ. usqe (QL/98, GL/75). In the Early Noldorin of the 1920s [ṣ] became [is]: ᴱ✶sṣt- or ᴱ✶pṣt- > ᴱN. hist “spits” (PE13/147, 163). This may have been the normal development of [ṣ] in the Early Period when it did not precede a labialized consonant (we see the same in Early Qenya). In the Ilkorin language of the 1930s, we also see that initial [s] became [es] before voiceless stops, which may be the inspiration for this Sindarin sound change.