Help!!

Have just bought second hand rear bumper to replace my damaged one. Looks like they may be different. Mine has four bolt holes in the boot and what look like two torx bolts under chassis holding it on and the puchased one seems to only have two bolts in boot and no torx under chassis. did Ford change design? Mine is a 89F RWD. If so will the new fit and what's the best way to fit?