Thanks to khj, cjd and jamesw, the mystery is (mostly) solved.

I did find the notes in the Scott that khj mentioned.
As stated, these stamps were intended as a commemoration of the Liberation of Macedonia. The 1 L dark green is valued at $1.75. Scott states it was prepared, but not issued. Other stamps in the series were released in 1917-19 (Scott 122-127).
According to Scott's note, yet another group of stamps commemorating the Liberation of Macedonia was intended to be released in 1915, but in fact was delayed until 1921 (Scott 153-157). Scott states the 50s violet was prepared, but not put in use. Value $1.75
Finally, Michel has both of these stamps (the 50s and the 1 L) with a release date of November, 1931.
So one catalogue is probably correct.
