Drop 2 III

The Drop 2 III voicing of Fbm7 stacks Fb, Cb, Ebb and Abb from the bottom up: the root, 5th, minor 7th and minor 3rd 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 Fbm7 are Drop 2 I, Drop 2 II and Drop 2 IV.