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Probably derived from Esperanto usage of ĉ and ŝ. (Can this(+) etymology be sourced?)
Proposal of this series for use in Chinese latinisation is noted as early as 1939 in Latinxua Sin Wenz literature (such as 《中國話寫法拉丁化指南》 by 王弦, page 78).
漢語拼音方案/汉语拼音方案 (Hànyǔ Pīnyīn Fāng'àn, “Scheme for the Chinese Phonetic Alphabet”) specifies that ⟨ẑ⟩, ⟨ĉ⟩, ⟨ŝ⟩ and ⟨ŋ⟩ may be used as abbreviations for ⟨zh⟩, ⟨ch⟩, ⟨sh⟩ and ⟨ng⟩ respectively, though these are rarely used in practice.