RGS: you might be getting yourself a great gun! The 'oil on water' is the original case colors. See
http://www.picturetrail.com/gallery/view...&offset=450The gun in that pic is an early NID with cocking indicators.
On the bottom of the chamber end of the brls should be stamped a number indicating choke: 4=Full, 3=Imp Mod, 2=Mod, and 1=IC.
NIDs are probably the last great deals out there but do the maybe 80 year old wood a favor and avoid serious boomers like the 1 3/8 oz Fiocchi GPs or 1 1/4 oz Federal Wing Shok High Velocity loads.
It's probably well past a good check and clean also-let us know where you are and we might be able to recomend a doublegun specialist smith near you.