I keep reading that the launch codes were set to zeroes in news articles but every technical document I read says the code was only used in Permissive Action Li

I keep reading that the launch codes were set to zeroes in news articles but every technical document I read says the code was only used in Permissive Action Link devices. As I understand it, these PALs were mechanisms that prevented the nuclear weapon from arming so even if the code was accidentally put into the weapon it wouldn't actually detonate until the ICBM launch sequence was initiated (by a separate process) and the warhead detonation mechanism fired. They are a safety feature that was added on to nuclear weapons starting in the late 50's, not some sort of detonation or launch control system. That said, the PALs weren't fitted to the entire US arsenal until the late 80's so lots of nukes had mechanical keys for decades. Hopefully they took the rehearsal part more seriously than the security of the arsenal...