Wrong Paint from Factory ?

On 71 and 72 Demon /Duster there is no number on the trunk support. It is on the package tray. I am presuming this is true for 1970 Duster as well.

OP is asking how to verify sellers story on the color mixup from factory, and is probably using it to justify why the car is painted FJ6 currently. As many would consider this partial year, high impact color as more desirable than Y8 gold, the seller may be adding a premium to the price the car is being offered at. If the owner insists the story is true, then it is on them to validate their story. If not, the color is Y8 Gold as per the tag.

Also, as OP indicated that car is represented as numbers matching, it obviously is being sold at a price considerably higher than a non matching model, and it is important that the OP verifies the truthfulness of the claim. Rad supports are easy to swap out. In fact, swapping in the dash, drive train and rad support is not uncommon to "save" one of these. The rear package tray number is "harder" to switch out and would be a number I would insist on seeing if purchasing.