Everything that was made by, or purchased by GM, has a casting number and a date code (it also had a temporary chalk or paint color code to let the assemblers know what part to use without actually picking up the part and having to read the casting number before using them). These numbers where required by GM for not only inventory control, but quality assurance.
Hobbiest noted this and define a numbers matching car to be the exact same part that the factory put in the car; or if it has a replacement anything the part that was used to replace the original has the exact same casting numbers and date code. There are books and web sites devoted to this subject as to identifying what your casting numbers should be, your application code denotiong whether it is the base motor, or an upgraded motor (as every car restored is a SS, with the biggest motor offered, as no one wants just the base engine), and the date codes decoded.
Some obses about this others couldn't care a flip what came from the factory and build their cars their way (back in the fifties they took this philosophy to the extreame building fantastic futuristic 1950 to early 60 looking cars that could only come from ther late fifties or early sixties when people though future cars would fly or have a built in home audio system in them). These cars were called lead sleds and most still exist, parked behind barns or out in front of bars in Mexico.