Drop 2 IV

The Drop 2 IV voicing of Fb7 stacks Ab, Ebb, Fb and Cb from the bottom up: the major 3rd, minor 7th, root and 5th of the chord.

It's one of the Drop 2 voicings: four-note chords in close position with the second note from the top dropped an octave, played over the root. The others for Fb7 are Drop 2 I, Drop 2 II and Drop 2 III.