I know this question was posted a few months ago; however I just came across the answer. Please keep in mind that there are several springs that came on the s197 Mustang. Year, model and options dictate which springs your car came with from the factory.
5R33-5560-BC: rear spring, standard, GT
rate: 24.9 N/mm, free ht: 361 mm, tag: blue
5R33-5560-AC: rear spring, special, Bullitt Mustang
rate: 24.9 N/mm, free ht: 355 mm, tag: light blue