Sears doesn't seem to like giving out the 148 (Soryu manufactured) models manuals from the mid 1970s. I have a pair of 148.13110s from 1975 and I had to buy the manual from somewhere online (can't remember).
Mine is a little different, but Joe is right on how to get it to get it to straight stitch. Top knob red dot at white arrow, lower knob 12 at white arrow would be a good stitch length.