Hang on a minute,
I would like to correct you...
Jack kept his head reasonably steady, but definitely not still
Check the third pic, check the trees in the background...Jacks head has moved appreciably (backwards)....
I'm not sure even that is steady or still
I've read that Jack Grout never really did that!
Young Jack didn't have that retarded center pivot. Look at my avatar!
You agree he always kept his head still on the backswing though right?
I think that his little pre-head turn set is what really kept his head still. As Jack got older, it looks like he overdid the still head thing.
My PC wallpaper is a picture of a young Jack N. obviously moving his head to the right. He really loads behind the ball in this picture.
When he revised his swing somewhat in 1980, he set up with his head more in the middle of his stance than before. In his instruction book the next year, he specifically talks about his head at address, and how he moved it more towards the center, from the right side.