In old cars, I think it's safe to say that if the various numbers stamped on the body and on various plates dotted around the car don't
correspond in an appropriate way, then something may not be correct with that vehicle. It could be that the vehicle was stolen and rebirthed or it can be an owner was trying to represent his vehicle as something it is not and thus make more dollars when sold on.
And by correspond, I don't mean match, i mean that in some cases otehr docs or the manufacturer may needs to be consulted since chassis number and vehicle (body?) numbers may be different in teh normal case. As such the manufacture may know if these parts were paired together at the factory (and thus genuine). Guess that may go back to when chassis and bodies were diffrent components which were married at the factory?
At least with modern (unibody) cars and the 17 digit VIN, things are much more clear. The VIN is usually stamped at the drivers floor, bottom left of the windscreen and on a sticky anti forge lable stuck somewhere (passenger b pillar or under boot carpet are common places). And the metal build plate in the engine bay also contains the VIN. In these modern vehicles, bodyies are single pies that is married to the driveline at the factory, hense whey the manufacturer needs to be consolted if one wants to check whether the current engine corresponds with what was installed at the factory.
Vehicles with 17 digit VIN's are much easier to check their authenticity that those old 70 and earlier rust buckets. It's a much more consistant numbering scheme than days gone by but for original engines, other things must be checked (1st rego papers, manufacturers records, etc)
At some stage, even VIN's will go by the wayside and it will all be done via laser etched QR codes
