Die-cast aluminum with steel set screws for adjustment lock and smooth or keyed shaft lock.  This will support a 1/2" belt and can be adjusted by backing off the lock nut and screwing/unscrewing one side to open or close the sheaves.  This is a great pulley for making minor adjustments in overall speed or power by changing the apparent diameter of the pulling by controlling where the belt will sit when operating.  I have personally used these for lapidary rock saws and grinders, old woodworking equipment and restoration work.