Earliest known usage of letter gamma “Γ” for reducible representation in group theory

Does any know the earliest known usage of the Greek letter gamma for showing a reducible representation of a group? This symbolism is commonly used in character tables in chemical applications of group theory, for example as shown here. Thank you in advance.

• Yes, some authors use an index i in Γ to show that it is irreducible. – M. Farooq Apr 9 '18 at 14:34
• I agree. The convention at least in the chemical literature is to use capital gamma for reducible and capital gamma with i as superscript or subscript as irreducible representation. – M. Farooq Apr 10 '18 at 14:06
• Citation needed. This excerpt only involves $\color{red}{\textrm{ir}}$reducibles (for which $\color{blue}i$ is an enumeration $\color{blue}{\textrm{i}}$ndex). – Francois Ziegler Apr 10 '18 at 21:39

Early examples are Burnside (1910, pp. 324-325; 1911, p. 271) where $\color{red}{\textrm{ir}}$reducible representations are called $\Gamma$, $\Gamma_1$, $\Gamma_2$, etc. (Earlier in (1901) he had called them $G_1$, $G_2$, etc.)
Speiser (1923, p. 104; 1927, p. 151) uses the same convention, but allows $\Gamma$ to be reducible.
Note that for them a representation is not a homomorphism to $\mathrm{GL}(n)$ (a map) but rather its image (a group). So your question morphs into: who first wrote $\Gamma$ for a group of linear substitutions? Now that goes back to at least Jordan (1870, pp. 221 sq).
• @M.Farooq Frobenius introduced the German word, Darstellung, nearly simultaneously (18 November 1897). I don’t think he’s been translated, but Curtis (1999) has a good exposition. Anyway, right: almost as soon as someone was in a position to call a representation $\Gamma$, they did. But it was really a group — hence, probably, the letter. – Francois Ziegler Apr 11 '18 at 17:04