This is an alternative “6-6 jazz font” version of Express Stave that has 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. In this version the notes for F and B have no center dash or spot, since their shape distinguishes them from adjacent notes.
For this version, Keller designed three original clef symbols. 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 the Express Stave page.
Source: John Keller. Express Stave, 6-6 Jazz Font version introduced February 2009.