Super stock springs originally came from the early development of the springs for the early 60's super stock package cars - max wedge then the hemi.

The early B Bodies had 20" front segments. Then they went to 21" then 22".

But the race program engineers realized the 20" spring had more strength so the program continued to use that length.

No significant A Bodies were in production until the release of the 68 Hemi A Bodies.

As for equal number of leaves per side - that's likely not a high performance pack - most all race spring packs had bias control built in them - more leaves on the front half of the passenger side spring compared to the driver side - and more total leaves on the passenger side compared to the driver side based on all the one side I have seen. Except the -002 and -003 Pro Stock springs which had thicker main leaves - I think .330" thick.