Early 39 deluxe fenders ( and early 39 mercury ) are more concave between the headlamp opening and the side.
Why they changed around mid 39 I don't know.
Late 39 deluxe are flatter in that area as are all 40 deluxe and 40/41 pickup fenders.
The only major differences I'm aware of between 39 and 40 are in the headlamp opening area.
I'm sure Kube will be able to add any extra differences i may have overlooked.
Chris.NZ.