Those look like aftermarket overload springs (the shorter leaves on top.) They were designed to contact bumpers on brackets bolted to the frame after the main springs deflect a bit. The idea is that the truck rides like a stock suspension, but after the rear end squats a bit from a load, the second set of leaves engage to keep it from squatting all the way down to the frame bumpers. None were offered as stock springs for a '53 3100.
