Brandon, I would think the 'helper' springs are almost certainly a later add-on.
As Erik mentions they inflate to increase the load capacity and to assist levelling.
I'm sure i've seen them for sale recently.
I do know AirRide Technologies sell a similar set-up but without the coil.


1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.