D 7alt, 5th inversion (Bb in the bass)
Also written as D7alt/Bb.
Notes, from the bass up
Bb (bass)DF#CEbE#
The same notes as D 7alt (Bb, D, F#, C, Eb and E#), rearranged so Bb is the lowest note. Use it to move between chords with smaller jumps across the keyboard.