That plate is the part called the apron. It serves 3 purposes. It keeps the wood from getting burnt by escaping gases and it defuses the gases, and is threaded to accept the screw that holds the catch plate. The catch plate is on the outside bottom of the stock directly below the apron. The catch plate is what the spur catch locks against. The apron is made of iron - this is a swedge piece .I've polished the one on my rifle because it makes it easier to clean. Also, I'm pleased to see the interest in this post. G.S.