IMHO, in the purest sense, a "numbers matching" car would have all of it's original major engine, transmission and rear axle components, as they are the most likely to be swapped. The block and transmission would bear the correct VIN stamping (on years that this applies), and all of the component date codes would be in the "accepted" range. The rest of the vehicle options and components would match the door data tag, buck tag, invoice (or Marti Report) and build sheet.
Most times, a car is considered "numbers matching" if at least the engine and trans bear the proper stampings. Early cars are tougher as the VIN stamping wasn't law until 68, so you have to go more on the date codes.
I'm not sure numbers matching applies to vintage Mustangs. From what I understand is that unlike GM Ford did not ID each major component of their cars with the VIN. So basically for a Ford you check if a certain part seemed likely (ie, options, stock parts) and then check its numbers which would denote its manufacture date and compare those to the car's build date. If in logical sequence, well that's about the best you can do.
Now for GM products I believe their numbers on major components do match which greatly increase its authenticity and value.
ford does not go by #'s matching, this is a term used by checvy/corvette restorers. it has been associated with mustangs although has no bearing on the matter, unless the car is a shelby who has an original motor which was stamped with the vehicle I.D. # on the block. the fact of the matter is, as long as the engine is the same size that the car came with & for the most part, the same year engine, you are in good shape. #'s matching refers to the engine being stamped with the same #'s designated to the cars body.
For Most Mustangs, it means nothing. If you wanted to be picky, then it would mean all body parts, engine, trans etc all have the proper date codes that SHOULD pre date the cars actual build date by a few days or weeks, perhaps maybe even a month or two but usually not longer than that and obviously never after. The engine and trans and interior codes and any other codes such as axle etc on the door plate should all be "matching" as to what is actually on the car. An A code vehicle with a 2 barel engine is not correct as it should have 4 barrel. Obviously in this case someone changed out the engine. One big scam is where the VIN plates on the car do not match what the door plate is. In this case its most likely a wrecked car and they put a new front end on. Fords are not the greatest when it comes to fool proof way of not being able to change the numbers. Now if you are talking about some of the high ultra perfomance mustangs like the K codes and Boss, they have I believe numbers (VIN) stamped on their trans and engine.
Chrysler and its brands were the most anal retentive about VIN numbers and they're stamped everywhere including the frame rear seats engines trans and lots of other places I don't know. Gm is very similar plus GM and Chrysler put a much more informative VIN plate on their respected vehicles than Ford so you could tell everything and anything from the plates. Hope this helps.
On my Cobra jet cars I've seen VIN #'s stamped on the block but not on anything else. I found a 70 M code Mach1 that appears to have the original sheet metal but I am having trouble with the date codes.
The engine components appear to have the correct dates (late 69) But no Vin#'s