It probably came from a school. My mother said they had to learn the Singer treadle machines (and all of the Singer attachments) when she was a home ec major in college, and that was post-WWII, so all the students thought that was completely ridiculous. The first high school she taught at still had a treadle machine tucked into the corner but it was only used as a novelty, for fun.
Amy Singer 15 manual will work.
http://sewing-machines.blogspot.com/...e-manuals.html