Numbers matching is where the casting number, the casting date, and the stamp pad numbers are correct for the car. Since the VIN derivative is stamped on the stamp pad (and that includes the year) you cannot just pick one up that would create matching numbers. Only exception would be if the original engine could be found.
We really have two separate issues here. One is the term "matching numbers" and the other is a car with the original engine. The matching numbers term was originally used to indicate a car that had the original engine. A car can have only one original engine, that is the engine install on the assembly line at the factory. Once that engine is gone it can never again be an original engine car.
A matching numbers engine refers to an engine that has the correct numbers. The casting number, casting date, and stamp pad numbers, as stated above, are all correct for that car. It does not mean that it is the original engine. It is possible to take a car that has an incorrect engine, go out and find a block with the correct casting number and casting date, restamp the stamp pad with the correct numbers, install it in the car and call it "matching numbers" since the numbers are now correct. And, true, it would be matching numbers but it still is not an original engine car, it is just a clone.
Matching numbers may or may not be an original engine car. But you could have a situation where an original engine car is not matching numbers. What? How? Say the owner had the engine rebuilt and in the process it was decked so the numbers on the stamp pad was removed. It now has the correct casting number and casting date but the numbers that identify the engine usage and the VIN derivative are missing. But it is still the original engine.
What is one to do? Two options: 1) only deal in cars that have a documented history or 2) forget the numbers matching nonsense and buy the car you like and drive it.
tom...