Don't know for sure, but it seems logical that the original intention of the design was to allow the soldier to hold the rifle by the grip with the right hand, place the stripper in the slot, push the rounds home with the left thumb, pull the clip, and then shift the left hand to the forend so the right hand can run the bolt home.
With a scoped sporter, I think I would balance the rifle on my right thigh while holding it around the action with my left hand. The right hand would place rounds on the follower with the left thumb assisting as needed to push them into the magazine.
I don't currently have a rifle with the thumb cut to practice that, but that can be fixed!
