There were definitely pumps in the 1870's but they were large industrial machines, steam engines or water/sewer movement. Baltimore was one of the first cities in the US to have a central heating system that could have had pumps too. Might have something to do with the waterworks industry or the Mills in Baltimore.
One other thought comes to mind: boilermaker. Not sure when that term appeared in the census, but people who worked in the metal fabrication for boilers, ships, and steel might qualify for this term too.