This is a small loose end of the foot that can be used to lengthen the foot also to adapt to the chosen middle piece. Tromlitz states that with the help of the last two devices five different middle parts are possible with a usable intonation. He does not mention what the range is that can be covered. However there are a number of flutes with five middle parts in collections and musea. The quality of the different tones on the one keyed flute is not uniform.
Some tones are clearly weaker than others.
On bad flutes clearly worse than on good ones. This difference in tone quality is used by composers for coloring the music in a particular way. These effects are probably partly reason for the renewed interest for the replicas of historical instruments. Also the tuning of the one keyed flute has some problems. Therefore they always are somewhat to close with respect to equal temperament.