mark, sign, symbol, number
號 centers on a visible mark or label used to identify, categorize, or count: a sign, symbol, or number. The idea of calling out or shouting (さけ.ぶ) is a separate, less common sense that may derive from using a distinctive call as a signal.
號 combines 号 (a simplified form of 号, meaning a call or mark) with 虎 (tiger), which likely contributes a phonetic element. The character originally expressed a loud cry or roar, later extended to signs and numbers.
The left side 号 looks like a mouth 口 with a mark above, suggesting a label or number. The right side 虎 is a tiger, whose stripes are like natural markings. Together, 號 is a sign or symbol used to mark something.
For コウ, imagine a tiger (虎) roaring a code (コウ) that serves as a secret sign. The roar is the signal: code -> コウ.