The bosses in the back were for the passenger footrest bracket but also, the pre 1979 Sportsters had a curved sheet metal piece that bolted there and wrapped around the bottom of the fender to keep it from vibrating and cracking. When they changed the back of the frame in 1979 they also went to a wider 16” rear wheel/tire combination which needed a wider fender and that bracket no longer fit.
The bosses in the back were for the passenger footrest bracket but also, the pre 1979 Sportsters had a curved sheet metal piece that bolted there and wrapped around the bottom of the fender to keep it from vibrating and cracking. When they changed the back of the frame in 1979 they also went to a wider 16” rear wheel/tire combination which needed a wider fender and that bracket no longer fit.