You can read about the S1C (first stage of the Saturn V)
here (dated December 1968). The thrust structure holds the engines and distributes the thrust evenly. There are 4 hold-down points on the structure, seen
here. They support the weight of the entire stack, and hold it down until full thrust is reached. Although I can't find a pic, it's actually a soft launch with the rocket pulling tapered pins through holes for half a second or so.