From what was explained to me was the brakes back then were asbetos which were highly corrosive the drum kept the dust from getting on the trim ring but the disc (like todays cars)dust would stick to the brushed rings and ruin them so they used the polished trim rings to avoid the corrosion.along with a different rim as I found out there is a different spacing from a drum rim to a disc rim. a disc rim will work on both a drum and disc but a drum rim will not work on a disc because it wont clear the 4 piston caliper.