この手紙を書いたのは何奴か。
Who wrote this letter? (literary or old-fashioned)
Standard kanji spelling; the character 奴 is read やつ here.
Mixed kanji-kana spelling, sometimes used for clarity or emphasis.
Compound of 何 (なに, 'what') and 奴 (やつ, 'fellow, person'). The exact historical derivation is uncertain, but it follows a common pattern of forming interrogative pronouns.