The Rickenbacker Model B lap steels from 1937 introduced a second control knob, which moved from the bass side to the treble side some time in 1939. Other minor cosmetic differences came year-to-year, such as an occasional plastic control plate, the exact type of knobs used, and white outlines on the frets. These were also the first Model B instruments to have different colors for the decorative plates on the face of the body, with black plates being the rarest.