Great pictures! Good lookin bike and a great find if you just scored it. My ideas on the bottom bracket cut out are these:
1) saves weight
2) intended for sealed bearing bottom brackets where moisture wouldn't matter
3) allows moisture to get out (and in)
Otherwise, seems sort of strange to allow the elemements access to the interior of the frame, unless the tubes ends are sealed by the barrel of the bracket housing. If one had cup and bearings for the crank, moisture would get to the assembly.