Rich and the Magpul guys would be the best to answer this question. But, to the best of my knowledge:
The strike plate is the aluminum piece bolted to the bottom tip of the tail piece, which is the lower half of the stock. The reinforcing plates are the aluminum brackets that clamp the upper half of the stock to the lower half (or tail piece). The M93A2 model tail piece has a plastic bracket molded to one side and uses an aluminum bracket to mount the other side. There were reports that the tail piece was breaking during clearance drills if struck at the right angle.
The M93B uses two aluminum brackets to clamp the tail piece on and is therefore "reinforced". If you have a M93A2, you can replace the tail piece with one from an M93B. Show below is my M93A2 with the M93B tail piece, reinforcing brackets, strike plate, and swivel sling stud brackets (which cost extra).