i can't believe i never really looked at the springs. I wonder if it's a continious progression or what the different rates are. From my pic you can clearly see that only the front is progressive though.

 

Logic tells me that the stiffers coils are closer together, if they were the softer ones, the coils would be binding and rubbing. Logic/theory does not always agree with real world practice. I may be wrong.

The Pro-Kit springs I have are black, the ones that Hypermark posted look to be anopther line and linear. I do believe that on the numbers painted on the springs, indicate the model and spring rate or rates.