The striker plate assy is held on with three large phillips screws and the plate they go it is in a cage so it will be retained when you remove the striker plate. Those screws are really hard to remove if they have been it there 60 years, a hammer drive impact with bit is the thing to use to get them moving after some soak time. The part in the door is removable from the inside of the door with the door panel removed. It to has those big screws it I remember and same soak and hammer manual impact tool will help on those.
