"Numbers matching"... I've begun to truly dislike that phrase because it's overused and very misunderstood.
Let's define a couple things first.... In 1969, the gubermint required the car manufactures to place the VIN (vehicle identification number) on several places in and on the car. You have the VIN on the dash, on the radiator yoke and under the trunk gasket. It's also stamped on the engine block and the transmission. It is also on the data plate on the left front fender well.
Part of the "numbers matching" package revolves around that data plate too. The paint color, interior, options, etc. listed on the data plate should match. That is a brown car should be painted brown. A bench seat interior should still be a bench seat interior.... All the options listed should be there and no more.
Then there's the build sheet. If you're lucky, the car still has the build sheet. That sheet is fragile in nature and stuffed under the seat, above the glove box or even in the headliner. It was nothing but scrap paper to the guys on the assembly line and mice don't discriminate about what they chew on, so that's why I say "if you're lucky". Some cars, like a lot of A12 cars, never even had one to begin with.
Now there's the matter of date codes. Just about everything that went into the car had a date code stamped on it. Guys go crazy trying to match the dates on replacement parts to come near the date the car went down the assembly line. That holds true with 68 cars too.
So... a true "numbers matching" car will be one that the engine and trans are original to the car. All options and paint will be correct. The data plate will be intact and hopefully there will be a build sheet. One step above that will have correct date codes on original and replacement parts.
The truth is.... The car is only original once.... and the more originality the car has, the more expensive it will probably be.
Remember, these were just cars once upon a time. Blow an engine? Find a wreck in a junkyard and swap the engine out.... No one cared about matching the numbers for the first 20 years of the car's life. These cars were hot street machines to some and drag cars to others. Lot's of "go fast" stuff got bolted on too and maybe the original stuff got saved and maybe it didn't.