It's a little piece of stamped steel with a dimple on it.
You are assuming it was there in the first place. The previous owner could easily have lost it and simply pushed the floorplate back on without the retainer.Whoever took it apart before you got it didn't make sure the nub on the retainer indexed correctly in the hole on the exterior base. ...
the magazine spring retainer plate fits between the mag spring and the mag base. it is flat and has a raised "button" that indexes in the hole in the mag base, thus locking the mag base in place.Thanks guys. We had some family problems so I didn't get a chance to read the latest reply's until today. I appreciate the pictures and schematic but I don't quite see how the retainer fits in the finger rest as the bottom is not flat