Something that has always puzzled me is the presence of what are apparently small spring perches directly above the main spring perches.  This picture shows these extra perches.  As I recently discovered, they are for helper springs:

This next image shows the helper spring pack above the axle.  Note the special U-bolts, which use the existing plate below the main leaf pack.  These U-bolts run side to side and sandwich the axle between spring packs:

This view shows the helper spring pack resting on the axle on my Jeep.  For the picture, I didn't disturb the stock U-bolts which run fore and aft and wrap around the top of the axle.  The helper spring pack has a pronounced curve in the front to follow the frame rail.  Rubber pads at the end of the helper spring pack contact the bottom of the frame rail under load:

These helper springs came my way with a spare axle I recently purchased.  With just a dog and two kids in the back seat,  I have absolutely no need for them and sold them to a friend with a Jeep-A-Trench.  Most later Jeep axles I've seen don't have these extra perches.   I'm curious when they were discontinued.