I was looking for the codes & spring rates for ALL the c4's.

These include fe1, z07, z51, fx3 etc. They are stock leaf springs on the c4 model.

TIA

The spring "rates" changed even though the same "option" was available in different years.

That is the key to the question. GM assigned the same 3 letter code to the same spring rate but put it in different option packages in different years.