The M-104 MXR Distortion + aka Distortion Plus aka D+ is a distortion guitar pedal designed by MXR and released between 1978 and 1979. The original stompbox did not have external power jack or indicator LED. Jim Dunlop bought the MXR licensing rights and currently manufactures reissues of this classic MXR distortion effect (with a power jack and an LED).
The MXR Distortion + uses germanium diodes and its sound could be defined as mild fuzzy distortion, like all of the 70's rock and 80's metal recordings that made this iconic pedal famous. The design of this circuit was reliable and used later on as a starting point for the M-133 MicroAmp which is, in fact, a un-distorted redesign of this (previous) M-104 MXR Distortion+ pedal.