Rear Helper Spring Option?

[MotoRod]

Looking to pick up a '78 GMC C15 with what the seller is calling coil/leaf rear suspension. Looking underneath, the coils sit on top of the axle and look to be more of a helper spring than anything. Does anyone know if this came as a factory option ?? I had a '83 C10 years ago and didn't have anything like this.

Thanks !!

 

[1979 c20]

Does not sound factory or dealer installed, but smarter fellers than I will tell us the real deal about this. You weren't given a picture, were you?