Roger Long wrote:
I haven't seen one of these. What does the flap do that a milk jug with
the top cut back doesn't?

Davis (I think) made a 2 gallon cleaning bucket where the bottom was a
flap that opened inward. It was really an "un-bailing" bucket,
because you could drop it overboard (attached with a rope, of course)
and easily fill it with sea water.