Bb 7alt, 5th inversion (F# in the bass)
Also written as Bb7alt/F#.
Notes, from the bass up
F# (bass)A#DG#BC#
The same notes as Bb 7alt (F#, A#, D, G#, B and C#), rearranged so F# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.