from everything i have read it is difficult to nail down the year of manufacture on these guns. Mine is serial no N077***V and it has a barrel code of PO. my question is would they have put a 77 barrel on a 78 gun?

Up to a two year "difference" between barrel manufacture date and receiver manufacture date is not uncommon. More likely with less popular models than standard field models. Also, barrel swapping is more common than you think. Remington's dating service accuracy based on serial number does appear to have improved somewhat since they tried to tell me two of mine did not exist.

yeah its a trap model with the 30" vented full choke barrel so it could be an early receiver. Thanks for the response

I imagine Remington orders and finishes barrels separate from the receivers, so not every barrel-receiver combo is a 1:1 fixed set.