The manual I have says "Stock Car Products #CS-300 (Standard) Lower end of spring covered with 14 1/2" of 3/8" heater hose" - perhaps this is what you've also run across. Regardless, "Stock Car Products" appears to no longer be in business. I assume the "CS-300" number refers to coil spring, 300 lbs/inch - that would about right, though my front QA1 coilover springs are 465 lbs/in. They're on an angle, though, so would be rated different than if mounted vertically as the rear springs are.
These guys (
Suspension Spring Specialist, Inc. ) list a bunch of different springs - all you need is a measurement for the "free length", the diameter of the springs, and the spring rating (lbs/inch). I've measured mine in the past, but didn't capture them. I'd have to get the car in the air and re-measure.