The gun has been chemically cleaned at some point, I say this because the pits are rust free. I see some rust forming in the grip area from finger prints and as long as the gun is bare it will be subject to easy rusting.
The gun is a family collectible and should remain in the family if at all possible.
Many of the markings are thin from natural use and acid cleaning, IMO you would not devalue the gun by giving it a finish of some kind.
I would not use any of the cold blue you find in the local stores, most will not last, Brownells has several alternatives that give a much longer lasting finish
http://www.brownells.com
I like some of the gray phosphate finishes that can be applied at home, they last almost forever and would keep the gun protected while maintaining a similar look.
Thanks for sharing your gun with us
Vern