There are several threads on this subject. Here's what Wikipedia says: 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.
Major components. These are parts such as the engine, transmission, rear-axle assembly, and frame of the car, with intake manifolds, exhaust manifolds, body panels, and carburetors 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.
If you are buying a car that is said to be numbers matching, you should ask the owner to identify what they mean - what components are original to the car. Verification can be done, but it is not easy sometimes. Matt