Bb 7b13, 4th inversion (F# in the bass)
Also written as Bb7b13/F#.
Notes, from the bass up
F# (bass)A#DE#G#
The same notes as Bb 7b13 (F#, A#, D, E# and G#), rearranged so F# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.