Boss SD-1 Super Overdrive
Schematic from the SD-1 service manual dated March 3, 1981.
Released February 1981 (according to BossArea.com)
Level, Drive, Tone. Basically adds tone to the OD-1, making it more like a Tube Screamer.
Retains OD-1 drive section (series feedback clipping diodes, asymmetrical) but adds a non-inverting tone stage. This makes the SD-1's output in-phase with respect to the input which is opposite of the OD-1.