There is a lot of talk about overlap in variations and changes.  A lot of beginning collectors assume that everything was done strictly in order, which just didn't happen.  I believe parts were placed in a bin and removed when needed.  Date and machining changes were simply ignored.

In mid-1943 they changed the frame by machining it differently.  More steel was left around the trigger axle pin for reinforcement.  You can see that because of a downward "hump" that is in the frame where the pin goes through.

About the same time as the addition of the "hump", there was a change made to the machining of the slide.  On early slides there are two cuts where you see the extractor.  The second, smaller cut was eliminated so it is referred to as a "late" slide.

Here is an example that has the late frame but still has the early slide.  Also, it has the "dusty blue" slide that is most common on byf42s.  I believe the slide was made in early 1943 and didn't get assembled into a pistol until late summer.  This is also a very early serial number gun to have a late frame.   By looking at this gun and the other byf43 I have in the "f" block you can see how overlap occurred.

Click HERE to see an example of a slide with the late configuration extractor cut.