My turn! Forgetting the different opinions on the ammount of step or if originally there was a step. Humor me and look at the pics, especially the better pics in Rifles of Colonial America #18.
Look at the repaired/replaced piece of the butt stock toe from both sides. From the cheek side it appears to be quite small. From the patch box side it looks larger. From the box side the wavy carving on that repaired/replaced piece does not look to me to match the rest of the wavy carving that surrounds the box lid.
If you straight edge the lower butt molding from the butt plate forward from both sides you also get a different impression
My best guess is that the repaired/replaced toe of the butt stock was reason for affecting the lower butt molding line. One side more than the other. Which leads me to think that the original step was not as significant as the molding tends to lead the eye now.