What is this pulley and bracket for?

When I bought my 1970 Dart 3 years ago, the previous owner had this pulley and bracket in the trunk. He told me that it was like gold and difficult to find. I know it is some sort of idler pulley, I think he possibly said for power steering bypass with A/C. It would be nice if some one could tell me what it was, and approximate value. It has stayed in the trunk and works well. Any help is appreciated, thanks in advanced: