Ha! I remember the milk stool! it was a set of scaffolding to bring the top of the rocket up to the level of the gantry, because the rocket didn’t require one of the stages.
I had a co-op job with NASA at the time. One of my co-workers was somewhere inside the crew launch vehicle when it went through a series of rather violent tremors. Needless to say, he was concerned that the milk stool might be coming unglued, so to speak. This was before the first crew launch. Turns out that the astronauts were practicing with the launch escape system (the slide cable from the tower) which was vibrating the whole Structure. My guess is that the milk stool just happened to be a bit resonant at that frequency.
IIRC that was for the Saturn 1-B?