In 1951 Kaisers could be bought in two different exterior trim levels, the Special and the DeLuxe. The bumper guards differ between them. If you have the Special, the LF and RR guards are the same and the RF and LR guards are the same. If you have the DeLuxe, all four differ and can't be swapped. The bumpers can be swapped but as was mentioned, the front bumper is drilled for the plate holder.