Given this plan was written in the fifties, knowledge about kinesiology has come a long way - The sit ups are problematic - situps in this style: legs flat, sitting up straight; not only does this not work your abdominals well (hip flexors, sure, but those are very strong muscles anyway) it also places a lot of stress on your back.
Rather than do the exercises as scripted, from chart two I plan to do “crunch situps” - knees bent, rolling first the shoulders off the floor, then the rest of the torso. Don’t hook your feet under anything. If you can sit right up, that’s great, but don’t worry if you cant.
EDIT: That’s what most people do - I think I mentioned it early on, to do crunches instead of sit-ups. I’m hoping that’s what folks are doing, anyway. Otherwise they’ll get a sore back!