I cannot vouch if they were used in the armed forces of any country as a training aid my limited experience as an army cadet in the 70's they made you hold the rifle (Mk III 303's) as intended in the prone position and if you could not achieve that then they excesized you to the point where you could achieve the desired position for the duration of the range courses of fire.
However in the regular services now with the equality & OHS stuff creeping in they may have a program in place to assist those that find supporting a rifles weight when prone an issue........