I'm not versed very well with pre 1967 V.I.N. numbers, but for what it's worth, the last 6 digits should represent the total production of cars coming out of that build plant.
That would include ALL Plymouths, Dodges, and Chryslers that were built there and all configurations be it 4 door, station wagon, convertible, 2 doors, etc.
681,196 is a high number, I think you are right in assuming that the car is a late build date one for sure.
I hope this helps,
Tom.
