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Many times these major components contain dates, casting numbers, model numbers, Vehicle Identification Numbers (VIN), stamped numbers, or codes that can match the original components that were on the car when it was new.
The term "number matching" (or "matching numbers") 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 be distinguished from an original car.
These are parts such as the engine, transmission, rear-axle assembly, and frame of the car, with intake manifolds, exhaust manifolds, body panels, and carburettors sometimes also considered.. Many times these components contain dates, casting numbers, model numbers, VIN, stamped numbers, or codes that can match the original components that were on the car when it was new.
These are parts that are commonly replaced due to regular wear and tear. Parts such as the interior fabric, paint, chrome trim, brakes, instruments, electrical components and wiring are considered minor components and generally do not affect the value of the car.
The numbers or casting dates on the major components of a car would be present and fall in a particular order. For example, an engine’s assembly date would be before the build date of the car, and the casting dates would be 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 be well in advance of the assembly date of the engine. Numbers and dates 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, because they are much rarer than non-number matching cars,  and are seen as a more accurate description of how the car was built.