69 383 Dart value ??

I agree the reason they used two sets of numbers on the body is in case of an accident. I would do as suggested and inspect and verify the core support was not replaced and that the quarter panel was it should be a lot easy to see that the quarter had been replaced.

Does it have any of the drivetrain? engine and trans should also have numbers on them. It should also have torque boxes in the rear at the very least.

The fact that one number doesn't match is not that surprising. If it had been hit earlier in life when quarters were not being made it would be normal to cut one off a car to replace a damaged one. The numbers from the lip would have been an after thought as they were not that valuable then.

As said verify the car against the build sheet. The color especially, traces should be found under the drip rail window seal, inside the quarters, inside against the firewall, if undercoated, scrape some off under the rocker panel. The color should match the factory color.