IIRC, bumper styles on most Mopars changed between 70 and 71. I'm thinking that 70 style bumpers used a jack hook that wrapped around the bottom of the bummper while 71 styles uses a slot in the bumper that the jack hook slid into. My memory is based on the 70 and 71 E bodies I've had, but would assume the change in jack styles may have been the same for all body styles.