Your Micro-Tron IV just got a little more fun! 2 alternate modules are available for fun and different voices of the classic filter effect.
"The 'Red' module has a slightly slower attack and decay time, which offers a longer “curve.” It is a bit chewier and is great for slower, swampy types of music. Also, in the “Vintage Mu-Tron pre-amp jumper mode,” the Red Opto-Mod can get a little grittier. This unit models an older Mu-Tron III which has drifted out of factory calibration."