Finally had a chance to look this up in an older Mopar book of mine. P2539334 and 335 are old, 70s era, Grand National oval track, stock car springs. Spring rate is 275#. That explains the thicker leaves, arch measurements, and 20.5" front segment. There is also a P2539336 in the set(that you don't have). The differing arches among the three would allow them to tune the arch to the track for the desired wedge to compliment the front set up.

For sake of comparison, the Hemi style XHD springs are around 160# rate and SS springs are around 180# rate. These are massive springs you have and a bit of a rarity to them. I don't think they would be worth much unless you could sell them to someone restoring an original, competitive wing car, and they would likely only be correct for a mid-length track or a highly banked short track. Some of the Super Speedway cars ran upwards of 400# rates on the rear.

If you use them on your '73 Cuda, you will need the special oval track hanger made by AR Engineering to keep the axle in the stock position. You could use SS hangers, but that will put the axle half an inch back and may require a longer drive shaft. It also will ride like an unloaded 1 ton truck, so you'd better step up to some REALLY good shocks to control them.