Well spotted, i didn't even notice it. I do prefer it with the flap for sure.
I found these pages of catalogues when i was trying to put a date on when the different "classic" filson bags came to life (duffle, field bag, original briefcase, etc). I can't seem to find anything older than these pages which make me think that they probably appeared when Stan Kohls took over in the early 80s and designed hundreds of new products.
If anyone has other info i'd love to hear more about it.