Bb 7alt, 3rd inversion (B in the bass)
Also written as Bb7alt/B.
Notes, from the bass up
B (bass)C#F#A#DG#
The same notes as Bb 7alt (B, C#, F#, A#, D and G#), rearranged so B is the lowest note. Use it to move between chords with smaller jumps across the keyboard.