I notice on lots of old photos of banjo players, they have some sort of a clamp-on metal device over or surrounding the strings between the bridge and the tailpiece. For instance, an old photo of Kirk McGee from Sunny Tennessee. Charlie MacCrow has one on the front page of Greg Earnest's web site on an old PB-3 diamonds and squares banjo. What is it called and what was it's purpose, and why don't they seem to be used any more?
Thank you.