Standard knee action, right leg straight in the backswing was taught by the 5 SK (one founder was A GSED).
They shows that not straightening the right knee and keep the spine angle will result in moving the upper body across, limited shoulder turn and extensive arm movement up.
Do you agree their theroy? I believe it is one effective way to swing a golf club but not sure about the right leg straight theroy. Many pros are not doing that.
Probably at that time they were teaching something very close to S&T (their own swing looks similar today but they say they teach more diverse patterns).
I think that what mostly keeps the head steady is the left tilt (thoracic bend) and the extension of the back. In S&T the straitening of the rear leg is more to keep the hips turning. In addition, the slant of the hips releaves lower back pressure.