'65 Formula S rear springs

So I have my "original" rear springs off my '65 Barracuda Formula S. The part numbers on both are 2403804. They are in very good shape and I will be reusing them with new rubber bushings. However, when I check the number in the '65 parts book, it shows Barracuda, Heavy Duty as 2495086. The "originals", 2403804, are 6 leaf per F/S spec, and fit and looked just fine when installed. When I search 2403804 on FABO, I come up with posts indicating these are for a '66 Barracuda Formula S. I am curious if anyone knows if there is a difference between the '66 and '65 Formula S springs. I suppose it is possible these were replaced at some point, but they are original Chrysler springs, not aftermarket, and can't imagine why they would have been changed out, unless it was a warranty exchange maybe. Any thoughts on how/why the '66 springs would have a different part number than the '65 F/S?