My thoughts on the plug that is just in front of the rear action screw is that it's either a wooden dowel installed there to repair an internal crack or check , or more likely, since it doesn't go through to the right side, a wood screw installed for the same repair, and the head covered with a wooden plug.
The recoil lug on Springfields is under the front receiver ring, as in the picture of Michael's Mielcarek