There was also the Looking Glass plane, constantly in the air, which could order attacks throught the Airborne Launch Control System if the National Command Authority was incapacitated.
I grew up in Minot, ND, in the 1970s surrounded with missile silos. One of the guys in our D&D group was a launch officer. He said that they were supposed to launch on a proper launch code, regardless of manner of delivery. If a rat swam out of the toilet carrying a piece of paper with the code in its mouth, that was good enough. That’s what he said, anyway.