The above two photographs show the location of a Rockstro F# lever on an ebonite Rudall Carte flute. Use of this Rockstro F# lever, which closes only the G hole, allows a perfect F# to be produced in all registers.
The normal RH3 C#/D# trill key has been moved to between the F key and the Bb side key. Furthermore, the Bb side key trills B to C, not A to Bb.
The Rockstro F# lever (devised in 1852) is named after its inventor, Richard Shepherd Rockstro (1826-1906).
This particular 'Rockstro's Model' flute was made in 1915, and is still in use today.
The above photograph and notes are as accurate as my knowledge on the subject permits.