If there is a mystery, it is an old one. The most difficult chants in the Graduale have always been a particular challenge for even the average parish choir, in any country. It is by no means only the XXth century that has sought to overcome this difficulty in practise by proposing simpler alternatives. The Chants Abrégés are an attempt to find a via media which would be useful in a typical parish.
Dom Gajard was chef d`atelier of the Paléo at the time, and under his supervision the booklet went to press. The tunes seem to have been chosen from various sources ranging from ordinary psalm tones simple or solemn (e.g. Introit psalmody) through melodic types for Allelujas etc. (e.g. Processionale of 1887) and Toni Communes for Gloria and Alleluja in the Mattins responsories (e.g. Liber Responsorialis 1895) to tones for Invitatory psalms or other simple cantillation formulae such as lections or Historiae Passionis, similar to those which Gajard suggested to Mrs Ward for the booklet of seasonal Mass Propers she published during the Second War.
So it is actually neither controversial nor mysterious, but rather a proposal to solve a "pastoral"problem in a meaningful way.
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