Tapered Black Universal Silencer. Pretty cool on a custom bike, but also seen on many scrambler and/or street tracker bikes as well.

They come with a mounting bracket + clamp.

Price per piece

Length: 16" / 406 mm
Inlet 1-1/2" and 1-3/4" / 38 mm or 44.4 mm
Fits either side.
Ideal for cafe racers and custom applications. 
dB-killer size: 45mm (not included)

The Sound?