Annie, I think you anwsered part of your question in your post. In the time period you refer to probaly 99.9 % of the people were from the farm, ranch or just out on the prarie and they were poor, but as for the different terms for mom, my granddad always refered to his mother as mam or mammy, he was born in SW missouri, in Oklahoma where I was raised I can remember a lot of the old people when I was a kid just refering to their mother as ma or mama, father was papa or pa. I would imigine it had to do some with where in the country you lived, less formal the further you traveled form the cities.