I'll concur with "under the warning labels", from what I've seen on my C-body cars. Like under the coolant freeze protectioin yellow label.
Agreed, the core support and OTHER locations on the body are all on bolt-on style metal pieces, BUT as these were more related to a nation-wide vehicle anti-theft tracing situation, I suspect the SO or other VIN-related number would have been stamped AFTER the basic car body was complete and ready for paint, rather than the stampings being done prior to those items being a permanent part of the vehicle body. Seems like there were about SEVEN locations throughout the vehicle? AND in places that would not normally be "in plain sight", usually obscured by labels, in weatherstrip-area channels, or other places which would take some getting to in order to find them.
Due to some of the normal aspects of vehicle assembly line activities, it MIGHT be possible that some core supports might not get the stampings. Nor would replacement parts for collision repairs.
When these stamped numbers were added to the body sheetmetal, it would have been reasonable to do the more popular A/B/E vehicles first. That's where most of the "hot rod" and "muscle car" vehicles were more prevalent. More theft-prone, generally.
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In those times, Chrysler was unique with their "plain open sight" Data Plates affixed to the under hood area, with the complete VIN and such on them. GM might have had their "Body Tag", but that was not nearly the same in the information it had on it; more to allow the bodies (from Fisher Body) to be paired with the appropriate chassis at the assembly plant) BEFORE a formal VIN was ever attached to the vehicle. Ford had their Cowl Tag with about the same information as the later Warranty Plates (of GM and others) on them. BTAIM
To me, the SO Number stampings might be better used to determine if the vehicle has had any major body/collision repairs rather than for complete documentaiton of the vehicle as being whatr it is supposed to be. Plus the aid for law enforcement for tracking stolen vehicles.
Just some thoughts,
CBODY67