NOS 340 EXHAUST manifold

I'm puzzled?? I have this NOS, never been used, covered with cosmoline, dealer tagged 340 manifold. Normally I would just sell it like all the other manifolds I sell but it seems to have a casting date of 1978 WHICH IS IMPOSSIBLE!!! From my research I know that these manifolds were made up to 1973 for the Dart, Challenger and Barracuda. It has the signature 1 7/8" exhaust hole like all the other (small hole) 340 manifolds I have so that part makes it a correct 340 manifold even though the year/date makes no sense. The part casting/number is also correct and very crisp just like the clock time. Can anyone make sense of this anomaly??? This would make a great addition to somebody's restoration.

PS - The cosmoline makes some of the casting numbers/letters a little unclear, they are actually very crisp (in person) but I don't want to rub the cosmoline off because it's probably been in this state for 40 years. I did remove a little cosmoline from the year date because I was looking for a "73" but found an "78" and couldn't believe it so I removed some of the cosmoline to make sure.

thanks,
treblig