Like all things in life, whether it be leafs, coils or whatever, "spring steel" is not all the same alloy, not the same quality, doesn't react all the same, have the same life span, etc. I wouldn't expect 2 spring assemblies made 20 years apart in 2 different countries to be close to being the same, despite having the same part number on them. The decision to use a set of springs out of a catalog vs. having something custom done for the specific application would depend on the seriousness of the effort, tuning decisions, etc.