After spending some time going through a big box of 40 years of used brake hardware and finding none of the smaller diameter springs, I looked through 20 salvage loaded backing plates and finally found one! I have a theory about these springs. I think that the smaller diameter spring was used early on in these brake set-ups and was found to be insufficient to return the shoes to a resting position after the brakes wore into the drums and most of the grease on the backing plates wore away, but while the brakes were otherwise fine. This would cause the brakes to heat up due to some level of friction with the drums and then Chrysler started using a heavier spring and specifying the heavier spring in service replacement kits which the other brake parts manufacturers copied. That's my theory.
