This is a vintage 1970’s Dan Armstrong Green Ringer electric guitar effects pedal. This unique ring modulator produces an octave above when only a singular note is played, however, a combination of two of more notes produces both the sum and difference between the multiple notes. Made by Musitronics Corp. (Mu-Tron) the company perhaps best known for their colorful envelope filter (the Mu-Tron III), amazing octave divider and great phaser units. The Dan Armstrong “Sound Modifier” small effects unit line, which were denoted by color, includes; the Orange Squeezer, Green Ringer, Purple Peaker, Red Ranger, the Blue Clipper, etc., all of which were designed to plug directly into the guitar or amp. Constructed from extruded aluminum square tubing riveted with painted cover plate, this extremely simple effects unit has a male hard-wired 1/4″ output jack and a single mini-toggle control. A unique ring mod/octave pedal, it’s simplicity enhances the tone quality.