Let's see if this makes sense:
The term “number matching” (or “numbers matching”) is a term used in the collector car industry to describe the authenticity of collectible or investment quality cars. Number matching generally means that a particular car still contains its original major components or has major components that match exactly the major components the car had when it was new. These "major components" are not always agreed on. The appearance of a number matching car likely could not distinguished it from an original car. A site by the name "numbermatching.com" has established a standard of specifications for a common definition of "number matching" and can certify them according to their company's definition.
The car's major components are parts such as the engine, transmission, rear-axle assembly, and frame of the car. Many times these components contain dates, casting numbers, model numbers, VIN numbers, stamped numbers, or codes that can match the original components that were on the car when it was new. In some cases intake manifolds, exhaust manifolds, body panels, and carburettors could also be considered major components.
The numbers or casting dates on the major components of a car must be present and fall in a particular order. For example, an engine’s assembly date must come before the build date of the car, and the casting dates must come before the assembly date of the engine because an engine assembly date (the date the engine was assembled, usually at a different location) could not be after the assembly date of the whole car. Engines are assembled prior to being installed in the car at the factory. Therefore, the assembly date of the car would have to be after the assembly date of the engine. Casting dates (the dates formed in the metal of a component at the foundry) could not be after the assembly date of the engine. And casting dates would need to be well in advance of the assembly date of the engine. Numbers and dates help track an accurate history of how a car was built and when and where the car and the parts used to create the car were made.
If a car has number matching major components it helps define how collectible a car is. Number matching cars typically will have a much greater value than non-number matching cars.
...they were also advertised as "correct mileage"
