In Noldorin it seems primitive [ls] became [lθ], but [rs] became [ss], as noted by David Salo (GS/§4.83, §4.183):

• ON. phalsóbe > N. faltho “to foam” (Ety/PHAL).
• ᴹ✶olsa- > N. oltha- “to dream” (Ety/ÓLOS).
• ON. persṓs > N. pesso “it affects, concerns me” (EtyAC/PERES).
• ON. tarsa > N. tars > N. tass “labour, task” (Ety/TARAS).
• ᴹ✶ters- > N. ters > N. tess “fine pierced hole” (EtyAC/TER).

This is distinct from the Welsh phonetic developments, where [ls], [rs] became [ll], [rr] (WGHC/§95iv). There are no examples of these sound changes in Sindarin, but there is no reason to believe they would not have occurred. There are no examples of ls, rs in Sindarin and Noldorin (except for Tolsirion on LR/128, elsewhere written Tol Sirion), so it is possible this remained an active phonetic rule.

Conceptual Developments: There is no clear evidence of whether or not these sound changes applied in the Gnomish or Early Noldorin of the 1910s and 1920s.