GSA CC & IKE hard plastic holders

Are they the same plastic? both issued at the same time so why would they be different? Yet almost all Ike's are toned from ugly to decent, from a little to a lot. I have not seen a toned GSA CC in an uncirculated holder. Only difference is 40% silver for one 90% for the other, would that do it?
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Comments
Keeper of the VAM Catalog • Professional Coin Imaging • Prime Number Set • World Coins in Early America • British Trade Dollars • Variety Attribution
The inserts, however, may have been ordered from different vendors at different times, with different plastics being used. Since many of the Proof Ikes discolor from the rim inwards, it would make sense that the inserts are the problem rather than the shells.