A 7alt, 4th inversion (B# in the bass)
Also written as A7alt/B#.
Notes, from the bass up
B# (bass)FAC#GBb
The same notes as A 7alt (B#, F, A, C#, G and Bb), rearranged so B# is the lowest note. Use it to move between chords with smaller jumps across the keyboard.