Bb 7alt, 4th inversion (C# in the bass)
Also written as Bb7alt/C#.
Notes, from the bass up
C# (bass)F#A#DG#B
The same notes as Bb 7alt (C#, F#, A#, D, G# and B), rearranged so C# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.