I was thinking it had to mean "Son of" something, so I guessed "Vulcan," Roman god of fire and the forge, since he worked with iron AND he was a fireman.
According to Wikipedia, the Sons of Vulcan was an American labor union which existed from 1858 until 1876. The union recruited puddlers, skilled craftsmen who manipulated pig iron to create steel. In the 1870s, it was the strongest union in the United States. It merged with two other iron and steel unions in 1876 to form the Amalgamated Association of Iron and Steel Workers—the forerunner of the United Steelworkers.
Given the time period, I'm thinking this is a solid guess.