I can see an application whereas the blade would be inverted and this cutout would serve well for running the length of the belly..."without" piercing into the intestine & guts to prepare for animal for butchering, similar to a "gut blade". That "has" to be the explanation. I have used gut blades by several makers, including "Bubba-Blades" (Warren Glover, custom maker from Georgia) making me one of my own design, and Boker Treebrand and a combo folder.
Best, Capt. Chris