Analyzable as *šim-ma, is to be equated with WS conditional particles (Hbr. ˀim, Arb. ˀin, etc.), which display a well-known shift *š > h > ˀ in (semi-)grammatical morphemes (Voigt 1995).
For the early Akkadian orthographic evidence against *ṯ in the prototype of šumma v. Frayne 1992:633.