That has to be a hand made cover sewn as a gift by somebody's partner (that last bit is just a guess). It's like the slip cover on a chair or couch, meant to protect the original finish. But like slip covers ... protect it from what exactly? And when do you ever allow the protected item to go au naturel?
Or perhaps it's a very sophisticated approach to controlling the resonance of the bass? The owner found it too lively, too bright, so it needed a mute.
Jimmy J
