According to an original parts book, there is no serial number breaks on the D-15 power director, so no early/late model.
The book only shows 3 plates on each side and all springs are the same part number, therefore the same length.
Your clutch is correct, the OEM manual is wrong. It appears the manual is referring to a D-17 power director.
I don't have dimensions for adjusting the shim packs, maybe someone else does and can supply the info.