Bogusław Julien Schaeffer (also Schäffer) (6 June 1929 – 1 July 2019) was a Polish composer, musicologist, and graphic artist, a member of the avantgarde "Cracow Group" of Polish composers alongside Krzysztof Penderecki and others.
So this triangle clef must be a modern notation and graphic product by this artist.
Schaeffer, Bogusław. Introduction to Composition. Ed. Ludomira Stawowy & Stefan
Ehrenkreutz. Trans. Jerzy Zawadzki. Krakow: PWM Edition (1976)
Added specification of locations for font-specific metadata to be installed on Windows, OS X, and Linux, to aid consuming applications in the identification of SMuFL-compliant fonts. Added recommendation that characters in ranges that will typically be drawn using runs of text (e.g. time signature digits, octave line labels, figured bass, and function theory symbols) should have appropriate non-zero side bearings. Reworked the triangular clefs in the Clefs range between U+E06F and U+E072 to match the descriptions given of their use by Schäffer in Karkoschka’s book. This involved changing the names and descriptions of these glyphs as follows: U+E06F was cClefTriangular, now schaefferClef ; U+E070 was fClefTriangular, now schaefferPreviousClef ; U+E071 was cClefTriangularToFClef , now schaefferGClefToFClef ; U+E072 was fClefTriangularToCClef , now schaefferFClefToGClef . Added z-style quarter (crotchet) rest to the Rests range.
In a dissertation Schaeffer is quoted:
“Within traditional conventions of composition, notation is
frequently substituted for composition; one composes not only with the help of notation
but often simply thanks to it. In new music the composer must tear himself away from this
convention and consequently from the notation.”