The various locations mentioned to you in part were uses on various manufacturers on various cars. For example vw bugs are stamped on under the rear seat on the linkage tunnel, some vehicles are stamped near spring perches and trans cross members. I've also heard of stamps on the panel lip in taillight openings and other sheetmetal parts viewable as the vehicle travells down the assembly line. I suppose it was important to get the correct chassis to body and correct motor crossmember into a unibody.
In the past I've read the evolution of vin was first used in the 50's to i.d. /match vehicle to title, later also to insure compliance with safety and smog requirements and of course to insure motor swaps were not using stolen, or older motors installed into a newer body for warranty fraud.
Motorcycles, VW Bugs and several other manufacturers and body styles are looked over closely during licensing inspections especially when coming interstate.
re mopars 6bbl's 2 locations, I agree.