Cremelo horses of the Ecole de Versailles

Thirty horses live today in the 17th century stables restored for the Academy of equestrian arts. Most of them are Lusitanian. Their cream-coloured coat (cremelos) and their blue eyes were particularly appreciated by Louis XIV.

Each horse works according to its level of training (basse école, haute école, saut d’école.) Each student rider is responsible for two horses: a confirmed one and a novice one.
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That is an intersting "cut out" on the saddle flap(fender for you cowboys) on this horse. I have never seen this before. I assume it is is to keep a riders legs in the proper position and to prevent the lower leg/spur from going too far back, like the knee blocks on the pommel prevent the upper leg from going to far forward for perfect balance. Does anyone know if it is a "new" innovation or something that was historically used, like the white buckskin saddles of the Spanish Riding School?