Goes to show weird things happen from the factory. I don't think the foctory was required to meet the 5 mph standards until 1974. The car you mentioned may have been produced right at the end of the 1973 model year. Like I mentioned I've only seen one 1974 Duster with shock absorbing bumpers in the front. Same thing could have happened to it. But then I've seen many 1974 Swingers with shock absorbing bumpers in both the front and rear.
