I think you've hit the source, more or less. So far as I know, it originated in the First World War, describing the miraculous return to base (or at least to ground level without killing yourself) after been shot up by the opposition. Flying by the seat of your pants dates from this period as well, I believe.

Both phrases were in use during the Second World War (or Great Patriotic War, depending where you come from) and have entered peacetime flying currency, thence ordinary speech.