It elevates these "instructional" stamps from secondary marks to primary objects of study.
McQueen’s most valuable contribution is his dating of the markings. He proved that the Jusqu’à system was not static. Jusqu-a Airmail Markings- A Study Ian McQueen
For a postal historian, a "Jusqu-à" mark is more than ink on paper; it is a map of a letter's journey. McQueen’s work allows collectors to: Jusqu-a Airmail Markings- A Study Ian McQueen