Mark is pretty well spot on.
Neither of the numbers on the vehicle is really the equivalent of a modern vehicle's compliance plate.
The number that appears on the ID plate tells you the sequence number of the specific model/bodystyle and manufacturing plant.
The number that appears on the subframe tells you the year (more or less), and sequence number of assembly and the plant at which the car was assembled. For reasons known only to GM-H, this number started at 1001, not 1, so you have to subtract 1000 from the stamped number to arrive at the actual assembly sequence number.
All these numbers make it difficult to ascertain the originality of any specific vehicle in any absolute manner. You actually have 3 number sequences to consider:
The Body Number, a sequence managed by the manufacturing plant
The Chassis Number, a sequence managed by the assembling plant, and
The Engine Number, a single number sequence managed at Fishermans Bend (the only engine-manufacturing plant), but sent in haphazard batches to all assembling factories, and presumably fitted to vehicles in the order in which they were removed from the crates.
However, resident Guru Ken has a heuristic method of resolving this complexity, based on his many years of researching FEs and FCs, and collation of vehicle data. Based on the information collected from real cars, original papers and so on, Ken can plot the data you have from your car, and make an educated guess as to whether the 3 numbers in combination vary significantly from the trends apparent in his vast collection.
Hope all that helps.
cheers
RET