This is a “jazz font” variation on Keller’s Express Stave: Pianoforte Notation that provides a subtle 6-6 pitch pattern through a variation in the slant of the noteheads. Notice that half of the noteheads are sharply slanted while the other noteheads lie flat. Keller recommends using this version of Express Stave when handwriting music.

It was when he introduced this version that Keller designed three original clef symbols for Express Stave. His F, B, and D clefs, shown here, replace the traditional bass, treble and middle clefs respectively. (Notice how the symbol incorporates the notes F, B, and D.)

For full details, see Express Stave: Pianoforte Notation.

Source: John Keller. Originally introduced in February 2009 using the notehead color pattern of Express Stave, Original Version. In that original “jazz font” version the notes for F and B had no center dash or spot, since their shape distinguished them from adjacent notes. Keller later came to prefer the notehead color pattern of the system he originally called “Express Stave, Reverse-Color” (now Express Stave: Pianoforte Notation), which was originally introduced using these “jazz font” noteheads in January 2010. In February 2021 Keller changed (“reversed”) the notehead color pattern of this “jazz font” variation to reflect his preference for the notehead color of Express Stave: Pianoforte Notation.