True matching numbers cars have EVERYTHING with casting dates on it that are built and assembled before the actual production date.
This includes the intake and exhaust manifolds, the carburetor, alternator, engine, transmission, rear end, yada yada.
On '68 and up cars, the engine and transmission is stamped with the car's partial V.I.N. on them.
The body on '69 and newer cars had the last part of the V.I.N. stamped in various parts of the body's sheet metal as well.
A true matching numbers car has the fender tag or data tag that matches the V.I.N. of the car and shows the major options on '69 and newer.
Keep in mind that the car's scheduled production date wasn't always the actual production date.
Usually the parts pre date the car's scheduled date by a few months.
A true matching numbers car is actually quite rare, because some minor components have been replaced due to normal wear and tear and maintenance.
A good example of this is the spark plug wires, fuel filter, alternator, water pump, and air filter.