Here's an original circa 1980 BOSS SG-1 Slow Gear.

The legendary pedal awaits you.

This is an automatic volume swell effect that removes the sharp "attack" of a plucked note, creating slow, bowed, or violin-like swells. It acts as an automatic envelope filter, fading in the sound based on input sensitivity, allowing for ambient or atmospheric swells without manual volume knob manipulation. 

Produced in Japan in 1979 as part of a new lineup of compact pedals. It was produced for less than three years, making it one of the rarest and most collectible vintage Boss pedals.

This is an early example, long dash and metal screw.   Shows some signs of wear.