Electro-Harmonix Octave Multiplexer
The Electro-Harmonix Octave Multiplexer is an octaver stompbox, produced and distributed by Electro-Harmonix, a well-known NYC-based stompbox producer. The first generation of the pedal, known as the "Big-Box Multiplexer" came out in 1970s (1976 - 1977) and was re-issued in a smaller box in 2001, being produced ever-since. It's build for both guitar and bass.
The Octave Multiplexer differs from other octave pedals such as the Boss OC-2 in that you don't turn have control over two octaves and a clean signal but rather have a "Blend" knob which adds the octave down as you start turning it up. The pedal also has a "High Filter" and "Low Filter" knobs and a "Sub" switch. "High Filter" controls the amount of the high-harmonic frequencies and adds some gnarly tone if you start turning it up. THe "Low Filter" is only engaged if the "Sub" switch is turned on and controls the amount of low-harmonic frequencies, giving you a subbier squarewave-ish synth-y sound.
The vintage EHX Octave Multiplexer was first seen used by Wolstenholme during The Resistance tour (and possibly even earlier). It's unknown which song he used it on exactly.