identify 3/4 springs 67-72

[496bb]

I think my 69 C20 has the stiffest springs possible. Unloaded, the back end is super stiff- and since I won't be hauling loaded most of the time, I would like to soften the ride some. PO told me he had loaded it down with rock and it barely sat down- even without inflating the airbags installed on the rear. I have searched the forum, and read that there were 2000lb or HD 3000 springs. Nothing left with the truck to help identify what options it has. How can I identify 3/4 rear coil springs, so I can determine which I have, and also if I do have the HD version- so I can identify the softer version.