The modification to the butt was to round-out with a rasp the undercut that the top strap sat into so that the top strap of the butt plate sat ABOVE the butt instead of being recessed into it. Now, if the butt was dropped onto the ground - as they inevitably are - there is no wooden undercut to get chiselled out. So simple that I wonder why noone thought of doing it like that originally.

I might be wrong here but the Mk2/1 guns were all fitted with the modified butt plates and butts during the early 50's FTR/conversion programme. It was 18,000 guns plus another 20,000 later if I remember correctly