Below are a couple of pictures of an old 16" wheel with a split rim and a M715 wheel lock ring for comparision. You can see from the first picture how much larger the M715 lockring is than the same size ring on the splitrim.
In inner lip of the split rim ring is only about 1/4" deep. The inner lip of the M715 lock ring is about 1.5" deep. Because of this extra depth the m715 lockring is held in place by the tire itself whereas the split rim ring is only held in place by the force exerted by the tire bead on the wheel.
The lip of the M715 lockring which covers the bead is also more substantial than the lip on the splitrim ring.