A long time ago, my dad, an accomplished competitive rifle/pistol shooter, drilled into me time and again, that direct bone support was paramount to a good shooting position. Every time you need to use a muscle to move the rifle into a position that "centers" your shot or "makes" your sight picture, you inject a variable into the shot you cannot reliably or consistently repeat. Your bones/skeleton is the only rigid portion of your body. The more relaxed your muscles and natural your position, especially in regards to that 8-10lbs weight at the end of those relatively small neck muscles, the more your bones are making the position and thus the shot ... and the more consistent the shot will be.
As for the picture above, I remember seeing more than one McMillan M14 stock at Dam Neck with all manner of ammo crate foam, EB Green (rigger's tape, 150mph tape, duct tape, whatever your group called it), even some really well-done fiberglass add-on cheek-pieces.