looking for a real fm3 demon 340 1971.

FM3 was also available as a 999 color afterwards.

Seeing that the red paint is the original unmolested factory paint, I think it sounds more like a mixup at the factory.
There are stories of black cudas with blue engine bays from the factory.
My guess is that someone mislabeled old paint on the paint line, started shooting the car, realized the mistake, and painted over it.