Meaning of "Date Code Correct" K-Code?

Date code correct means no one can find an inconsistency in the date codes and stampings. That still leaves a large margin for an engine to be date-correct, but not the original. But, the difficulty in getting all the dates correct (within acceptable parameters) is quite high, so non-original engines can often (not not always) be discerned by careful examination of all date codes and stampings.

Having said all that, there will be cases of date codes seeming to be too far off to avoid suspicion, yet still be original. In most cases this will be related to parts with date codes well before what might be expected, or the car being delayed until after its scheduled build date.

For example, I have a 1965 Fairlane K-code built in April 1965. All the date codes on the differential are late 1964 production. It would seem that the differential was incorrect. But, upon disassembly, I found that the axle housing had a factory repair due to a fold-over in the center section stamping that probably caused the housing to leak. The fold-over caused a crack, which had been professionally welded, and then machined. It was likely this housing was pulled from the line during an inspection (when it leaked), repaired, and eventually put back into the production line. It is the professional repair that ties the span of time and makes this housing highly likely to be the car's original. I just can't prove it, but I can live with my explanation with the photo-documentation of the damge and repair.