A great many '40s had the year digits together on one stamp, like Flathead Ed and Eric's uniformity suggests.
But that doesn't mean that all did.
Note the sharp point at the top of the "4"
The earlier font had the point truncated.
This collection suggests that the serifs were dropped on big twins in 1937 somewhere between 1760 and 1765.
Perhaps old stamps were dedicated to the military, along with old stock of cases. It would be interesting to know the belly numbers, of course.
PS: photo credit to Panic for some of the examples.
PPS: Do these thumbnails open up for everybody?