Dinodave,
This is the interesting part. The research to determine exactly what your antique is and what "original" really is.
Model changes were initiated by headquarters, but the individual factories didn't implement them on a given date. They had different amounts of inventory and always wanted to use up as many of the parts on hand as possible. So one or more models were designated to use the old parts longer. Maybe the old dash boards went to the 1-1/2 tons and the 1 tons got the new, same for doors, axles, etc.
The engine production line would have shut down for a while to retool for a change and may have started up producing months before the model year changed or not.
Lots of research , lots of time, but the answers will come.
Remember, back then they weren't anal on record keeping as they are today.
I have a "1941" Packard 120, the model change came in 1940-1/2.
Since the dealer still had it in stock when the 1941 models came out, he took a hand stamp and changed the ID plate from 40 to 41.
