Here is an Ebay auction for a used pair of rear leaf springs for a 65/66 Hipo by Ebay member "shanty137" with a starting bid of $999.00 and a Buy-It-Now of $1,299.00:
This auction only includes the last leaf in the pack that have the engineering numbers as shown
You are bidding on 1pr of used original Ford rear bottom (last) leaf springs. These are the last leafs in the leaf spring pack that have the Ford numbers. The numbers C4ZA-5556-S were used on 1965 - 1966 K code fastback , 1965 - 1966 Shelby , and 1965-1966 A code with GT handling package. With the date code of 137 E5 , I believe it decodes to the 137th day of 1965 , These could benefit from a deep cleaning or media blasting. Most people add this leaf with the ford numbers to a new set of Eaton leafs to make a complete set of leaf springs.
Extremely rare and hard to find.