If you are not the original owner then there is no way of telling what someone did. Maybe something happened to the OEM motor and they were lucky to find another just like it. Would seem it had to be changed way back when
.
The engine is a common Camaro engine and there should not have been a shortage that could be expected with some hi-perf option.
Have you found any VIN derivative on the engine pad? Something like 7L118264 ?