Hello Don,
Those look to be from an older parts book. One where they ran out of parts and sub'd the over the counter parts that were close.
As example EK was 3955710 but since they did not have that one, they list the 3955709. After 1970 they only listed two service front springs for the Nova.
You can see the full list of Nova Springs here along with a few calculators.
Spring Calculators and Charts
No Chevelle ones yet, but in the order book when I find time.
Greg