Tonight seemed like a good night to fill in the South Dakota State Census information for my grandmother's family of origin. The state census was taken every five years, and FamilySearch.org has the actual cards filmed, so I bypassed Ancestry and went directly there.
Because the cards are arranged alphabetically, I needed to look up each person in the household separately, which was no big deal. I happened to be working backward, from 1935 to 1905. The mystery is in the 1915 census.
I found my great-grandfather, Peter Christensen, Card #141, first. Then his wife, Ella, Card #142. Then I found my grandmother, Lillian, age 3, Card #144, and finally, Raymond, age 1, Card #145. The next child to be born would be Clarence in 1917. So, who is enumerated on Card #143?
Perhaps for some reason the card needed to be destroyed and does not exist.
The most natural supposition is that Ella's mother, Alvilda Monsen, is on that card. Alvilda arrived at New York City, aboard the ship "Kristianiafjord", on 16 May 1915. She was bound for Huron, South Dakota to the home of her son-in-law, P. C. Christensen. I do not know the exact date the Christensens were enumerated, or if Alvilda made it there in time to be counted among the household.
Family Search did not list an Alvila, Alfhilde, Monsen, Monson, nor Munson in the index. Ancestry did not, either.
So, the question remains: Who is on Card #143?