This system is identical to Cornelis Pot’s Klavar with two exceptions. First, the pitch axis is pitch-proportional as in the Mirck variant of Klavar. (Unlike Mirck, however, Dekker retains the vertical orientation of the original Klavar staff. The above image of the chromatic scale rotates the staff 90 degrees for display alongside other systems.) Second, the placement of noteheads above or below the stem has changed to a 6-6 pattern instead of a 7-5 pattern. The noteheads for C, D, E, F#, G#, and A# are placed above the stem, and the noteheads for C#, D#, F, G, A, and B are placed below the stem. This pattern makes this version of Klavar correspond well with a 6-6 keyboard that has 7-5 key coloring, such as a Jankó piano. It also means that any interval or chord has one of two possible shapes, regardless of transposition, as seen in the illustrations by Dekker below.
The following image compares the Dekker version of Klavar with the standard version (a.k.a. “Klavarskribo”), and illustrates the greater consistency of interval shapes in the Dekker version.
Dekkers’ 6-6 arrangement of noteheads with respect to the stems is similar to that of Isomorph, as can be seen by comparing Dekkers’ major chords above with Isomorph’s in this PDF. The difference is that Isomorph uses a 6-6 staff and a 6-6 coloring of noteheads, whereas Dekker’s staff and notehead coloring are 7-5.
Source: Antoon Dekker, posting to the Music Notation Project Forum on July 21, 2015