What is this pulley and bracket for?

The 67 system is not the same as the 70 and up. It all goes back to the cast iron water pump, 69 and earlier. On that setup 1 belt runs alt, water pump to crankshaft, a narrow 13 series belt. The AC belt (1) a 17 series belt, just goes to the crank and the adjustable idler and a smaller idler that runs on the backside of the AC belt that mounts in front of the PS bracket. Then 1 belt that runs the PS. Any one can be run without the others. In other words the 69 and earlier idler is not the same as the 70 up one.