Ab+Bb trill with extra tonehole / without key

What: extra tonehole is sprung to open but held closed by the Bb-key (R-t) and by the G#-key (R-4f); thus, it cannot open unless both the Bb- and G#-keys are opened/pressed.

Ab+Bb trill: w ● ● ● / Bb ● ● ● Ab.

Note: extra tonehole will open for high-D (improving intonation and slurs to high-D) if fingered:

d ○ (½) ● / Bb ○ ○ ● Ab.