UPDATE:

We're glad to inform you that we have fixed this issue. You should now be able to fit the Stage 6 upgrades to the correct car model. Thanks for your patience and understanding.

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We've noticed that some players are having issues with the lock-in cars for the ongoing Duality Cup, wherein players did not get the correct Stage 6 upgrade. There is also confusion regarding the accurate car model to use. Our team is now looking into this, and we'll keep you posted once it's fixed.

We are sorry for the trouble this has caused, and we appreciate your patience and support as we work this out.