If you had two different lengths as well that would seem to indicate it's original. Those bolts are just very unique, a carriage bolt with a shoulder at the end of the threads as well, not to mention the size. My rubber bushings seemed to have been replaced at some time and the bushings for the longer bolts were wider but that would mean that the factory wanted the shackles to be canted. Why?? On mine the longer bolts were on the spring eye, not the frame.