The shape came from CISM/ Club shooting competition design development in the 70's. The competition style never really caught on in North America, but the AMU among others did head overseas to compete.
You'll see that there are more commonalities than differences in stock design in the 70s and 80s between Anschutz, Tanner, Sig (STR200), CG (CG80/FSR89), Unique, Hammerli, and others, and those shapes became useful for Running Boar/ Running Moose as well.
In a way it does go back to whoever it was who bolted on a second piece of wood to the grips of their M96 for the Feldskytte/ Stangskytte and then it just progressed from there through the M28-76, CG63/CG80, etc.
The Finns in general caught on early to stock ergonomics and have really been frontrunners of design since the 70's