A 7alt, 3rd inversion (Bb in the bass)
Also written as A7alt/Bb.
Notes, from the bass up
Bb (bass)B#FAC#G
The same notes as A 7alt (Bb, B#, F, A, C# and G), rearranged so Bb is the lowest note. Use it to move between chords with smaller jumps across the keyboard.