I don't know anything about the mark Nitro King, but I can tell you that it's very unlikely that the shotgun was for blackpowder shells.
Nitro, as a phrase to describe gun powder, didn't exist until smokeless powders came along commercially starting in the 1890s. Nitroglycerine was (and still is) a component in many of these powders. Most people were also familiar with nitroglycerine as an explosive, so it was a natural marketing leap for companies to attach the name nitro to their ammunition to denote its power.
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