Price listings here are for the original version of this effect
from the late 1970s and perhaps into the early 1980s. These have
been reissued and were apparently manufactured between 2005 and 2008.
The reissues included a “sub” switch and this version is apparently discontinued. A completely new version is also available from Musician’s Friend (and elsewhere).
This from a 1970s EH brochure:
“Now synthesize a note one octave below the one you’re playing. The Octave Multiplexer
literally allows you to convert any standard electric guitar into an
electric bass — making possible lightning fast bass runs. An
unbelievable extension for horns and trumpets. It explodes the tonal
capabilities of these instruments into the range of the bass and
baritone horns. Five filters allow the user to shape the harmonic
content of the new note from fuzz bass to a pure deep organ bass. The
Octave Multiplexer is fantastic on voice too; it makes any singer sound
like Ike Turner.”