Hey Brian. No, the oil cooler lines (and most of the rest of the fuel & other hoses) had been disconnected in 1976. The original owner thought he had blown the engine, thus started to unhook the engine but never got any further. The car sat like that until I bought it 35yrs later. Turns out, the engine wasn't blown or damaged.... which is kinda funny now....
Anyway, yes, I'd love to know why he cut the hole we're discussing, but I have no idea. I was hoping the factory had done this and by posting the pictures to this forum, someone might be able to shed some light on it. In the end, I think we've concluded this particular hole wasn't created by the factory line workers. Just for the fun of it, I added one last photo, taken from the engine bay side, to provide a different perspective.
The mystery remains.