Ḿ
Ḿ, ḿ is a letter in Chinese pinyin. In Chinese pinyin ḿ is the yángpíng tone of “m”. It was also used in an old version of the Sorbian alphabet and in older Polish.
This letter is also used in the Bube alphabet, a language from Equatorial Guinea. It is also used in Võro, a language of southern Estonia.