Although I'm a bit out of my element here I seriously doubt this was done at the factory. If it was returned to the factory to have it done it would have a barrel code followed by a "3". Remington didn't do much in the way of nickel plating during that period, whatever the finish it was done a long time ago and poorly. It almost looks like it was electroplated, been exposed to 500 series rifles for a long time and they have all been blue or Parkerized. I'll bet its fun shooting it outside with the Sun reflecting off the barrel......oh well, its an interesting piece.
Hopefully you will get a more educated response from someone!
Jim Peterson
Charlotte, NC