MXR Distortion II pedal, from "Stompland"
Distortion pedal made from 1978-1984, with 4 dials for drive, resonance, filter and output. Apparently designed to emulate the sound of an overdriven amp pushing a 4-12 cabinet. Used for the leads on Siamese Dream, run through a 70's Op-Amp version Big Muff.
Featured in a video of Billy's huge collection of guitar pedals called Stompland. The SD settings were still clearly marked in (knobs at approximately 5:00, 11:00, 11:00 and 2:00).