Μ μ
From LSJ
English (LSJ)
μῦ, τό, Ion.
A μῶ Democr.19: indecl., thirteenth (later twelfth) letter of the Gr. alphabet: as numeral μ = 40, but μ' = 40,000.—In Inscrr. M stands for μύριοι or μυριάς: hence <*> for πεντακισμύριοι or πέντε μυριάδες, 50,000.