This is the accidental classic Radio Shack Electronic Reverb Control. It was produced as an all-purpose "reverb" for PAs, keyboards, microphones, and yes, even guitar. But the unit's relatively low headroom has made it a popular overdrive unit for distortion-savvy guitarists. 

Although marketed as reverb, the unit is actually a bucket brigade analog delay with a Panasonic 3207 chip. It's the same chip that is in a lot of vintage classic analog chorus and flange pedals. In this application, it gets up to 95ms of delay, which is adequate for slapback echo.

But most modern guitarists use these boxes as a console preamp-style distortion. It has mic level and line level inputs and outputs (mic level is 1/4", line level is RCA). The sound changes depending on which input/output is being used. Here's what the sliders do:

• MICROPHONE: This is the input gain of the unit. Unity gain is around 4. Moving the slider past that point starts clipping the input, giving warm overdrive, all the way to ripping square-wave distortion.
• DELAY: This sets the rate for the delay. Turning it up gives you longer echos, up to 95 ms.
• REPEAT: This sets the number of repeats. 0 gives a single echo; 10 gives near infinite echoes. 
• DEPTH: This is the level for wet (delay) signal. 0 is 100% dry. 10 is 50/50 wet/dry.

This Electronic Reverb Control is in fair condition. The sliders are scratchy when being adjusted. There was corrosion in the battery compartment. But the battery snap has been cleaned and still works. You can power this with a 9-volt battery, a Boss PSA-style ac/dc adapter, or your standard 9-volt pedalboard power supply. Sold as-is.