Matching Engine to Production Date - Pre-'68

Clarification:
Depending on the model and/or dealer, it could be an "as delivered" vehicle, but not "as built".
It's a stretch for sure, but someone buying an early GSS from Mr. Norm, for instance, could very well have engine date codes that do not sync with the body's build date; since they were hand "rebuilt" at the dealership with different engines... I'm sure some Plymouth dealers could have done the same thing.
Uncommon situation, but I suppose you could say it's possible, given the info you gave us. But unless it is a special non-standard build, it ain't original.