Lines an Octave Apart, 1 Line per Octave
From the Guided Tour: One system uses only one line per octave, giving it the least dense appearance of any system, but it also uses the most ledger lines, with 7 of the 12 pitches requiring them. It uses notehead color and shape to help differentiate between adjacent pitches.
Untitled by Nicolai Dolmatov

Each note is either a traditional oval notehead or a small "x", with six of each shape per octave. Traditional stems, flags etc. are used. The standard staff spans two octaves, consisting of three horizontal lines widely spaced (approx. 12mm apart) with each line representing C. Ledger lines representing E and G#/Ab occur as needed.
Earliest documentation: 1995 (Music Notation News)
Source: Directory of Music Notation Proposals, section/page: 10/13, 11/1, 13/27
Manuscript Paper: Discontinuous Staves Continuous Staves