I've seen them before too but not specific to any plant, vehicle or engine options. .
Sure was easy enough for it to happen being a human on the other end of the gang stamp and remember as long as the number was legible, all was Ok on the line. .
Is it a whole car you're looking at or engine only? .
If a car, how's things look when you compare the fonts, spacing etc. w/ other Vin locations remembering the same gang stamp was used? .
If an engine only, aside from the upside-down thing, how's the fonts, straightness etc. look otherwise? .
Is the window of Vin production date good when comparing to the engine build date? .
If you have concerns also have time before you have to act, try to find a close Vin and compare fonts etc?
Gotta' think if someone was pulling any restamping tricks, why draw attention to anything by stamping upside-down? .
~ Pete