I clicked on the link and ended up talking to a guy in California who informed me the "Back to Black" product was carried in automotive parts stores. I drove a short distance to O'reilly's and purchased a bottle.
It worked, more or less, on my rifle stock.
As I stated in my above post, when I purchased the gun, it had one of those stretchy black cartridge holders on the stock. I removed it, and underneath, the surface of the stock appeared to be oxidized: it looked whitish. The area was the shape and size of the cartridge holder. The "Back to Black" restored the area to its black color, but the surface still has a different texture from the rest of the stock--a texture like it was damaged by oxidation. The product made the rest of the stock look beautiful--a shiny black. Thanks for the tip.
I'm left wondering why the stock was damaged by the cartridge holder?