Ford 'shared' as many components as possible in engineering the '55-'57 T-birds as well as other models. This was particularly true of mechanical parts. Two reasons, to speed-up product development for the 'Bird and to reduce production costs particularly for a vehicle that would not be sold in the quantities of their other models.
That said, FoMoCo often found it necessary to have unique parts for different body styles like passenger cars, station wagons/ sedan deliveries, and Thunderbirds. Sometimes they also specified differences in the parts used for certain models not because of component dimensional differences, but because of other factors related to a model's weight or anticipated use. Examples of this would be different part numbers for Police Interceptor, convertible, or Victoria model cars.
The part numbers assigned to the leaf springs for the three years of 'Birds were
B5S 5560-D for the 5 leaf spring for the '55 'Bird
B6S 5560--B for the 4 leaf spring used on the early production '56 'Bird
B7S 5560-A for the 5 leaf spring used on the later production '56 and all '57 'Birds
The 'S' in the part number prefix signifies that there is something unique about those leaf spring numbers that means they were a Thunderbird specific part. Going deeper on this, the '55 'Bird used a different length leaf spring than the '56 and '57. The 4 leaf springs on the early '56s weren't stout enough, so they moved to the 5 leaf spring for later '56 and all of '57.
For the '54-'56 Ford passenger car, a 5 leaf spring was supplied part no. B6A-5560-A (this number superseded the original '54 part number)
Also, for the '55-'56 Ford passenger car and Police Interceptor cars, a 6 leaf heavy duty spring was used, part no. B5A-5560-C
Hope this helps.
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York