The factory etch primer on my 69 is a dark greeny grey colour. Very tough and adhears very well. Weather it is original or not you probably don't need to completely remove it. Remember that these cars rust from the inside out. Any extra sanding or grinding you do to the surface just eats closer to that rust on the back side making the car rust out sooner. No primer or paint on the insides of panels from the factory. All inner surfaces probably have some rust working it's way to the surface. I would feather out any chips, rust spots and repair areas with 80-100 grit paper then finish the entire area with 180-240 grit before priming.