IMHO....I think you may be looking for a "Date Correct" reference.
If you are replacing an original part with a correct original part the date codes should preceed the build date of the car by 30 maybe 60 days hopefully.
In this stuation I couldn't refer to the part as "numbers matching".
Ron, as Waynew indicated there are really two things here. To the purists, numbers matching means the car still has the various components with which it was originally built......engine, transmission, rearend, etc. They would also consider it should have the original carb, intake, exhausts manifolds, accessories, paint and trim matching the cowl tag, etc.
Most cars that are called 'numbers matching' are not and should probably be referred to as 'date correct.' This means the various components are not necessarily all the originals, but are correct parts dated within acceptable time frames for the build date of the car. Chevy By The Numbers indicates that 1-3 months prior to the build date of the car is acceptable, although this was frequently extended by labour disputes, strikes, raw material shortages, etc.
Hope this helps.
On newer models you can match the motor numbers to the VIN. I did not think that you could do that on the early Novas. There seems to be no connection to the VIN on the motor in my 65 although it is a correct Nova motor. Am I worng on this, or is it possible to truely tell the difference between date correct and "numbers matching on the older Novas?