No, not his mother-kink (that's been quite well documented):

I think I need to buy a "kangaroo pouch machine" musical gadget ...

Free Music and Grainger’s machines

In Australia, Grainger is remembered chiefly for his musical innovations and for what he called “Free Music”. He first conceived his idea of Free Music as a boy of 11 or 12. It was suggested to him by observing the waves on Albert Park Lake in Melbourne. Eventually he concluded that the future of music lay in freeing up rhythmic procedures and in the subtle variation of pitch, producing glissando like movement. These ideas were to remain with him throughout his life, and he spent a great deal of his time in later years developing machines to realise his conception.

Free Music is melodic (polyphonic), making use of long, sustained tones capable of continuous changes in pitch. No traditional form of notation exists to describe it in detail. Grainger's own scores were originally notated on graph paper, with an individual trace for both the pitch and dynamic changes of each note. Free Music assumes a moving tone, precluding any harmonic stability and working with Free music is difficult since almost every basic assumption about musical relationships and method must be ignored. Free music requires the abolition of the scale and its replacement by a controlled continuous glide.

Grainger resorted to the use of machines because human performers on traditional instruments were not capable of producing the wide range of "gliding tones" with the necessary control over minute fluctuations of pitch. The machines were not intended as performance devices. Rather, they were designed to allow Grainger to hear the sounds he composed. He insisted on hearing his compositions before allowing them to be published, and often went to extraordinary lengths to achieve this.

His most famous machine is the "Hills and Dales" machine, described by Grainger as the "Kangaroo Pouch method of synchronising and playing eight oscillators" (on display in the Grainger Museum). Commonly known as the “Kangaroo Pouch machine”, it consists of a large wooden frame approximately eight feet tall, housing upright rotating turrets left and right (the "feeder' and "eater" turrets) and between which a large paper roll is wound. This roll consists of three layers: a main paper roll 80 inches high, across which eight smaller horizontal strips of paper (or subsidiary rolls) are attached front and back. The top edges of these subsidiary rolls are cut into curvilinear shapes (the hills and dales) and attached to the main roll at their bottom edges, each forming a type of "pouch". As the turrets are rotated clockwise, the undulating shapes cut into the rolls move from right to left. Eight valve oscillators are mounted onto the wooden frame, four at the front and four at the back, as are eight amplifiers. The pitch controls of the oscillators are attached to levers, connected at the other ends to circular runners, or spools, which "ride" moving rolls. The volume controls of the amplifiers are operated in the same way. Thus, the pitch of the oscillators, and the volume of the amplifiers, can be accurately controlled by carefully cutting shapes into the paper rolls. The large size of the machine is necessary to maintain accuracy of pitch control. Because the valves changed characteristics as they aged, the machine needed to be recalibrated after around three hours of use.

Grainger’s final machine was perhaps the most sophisticated. It too worked on the principle of a moving roll, but this time made of clear plastic. A row of spotlights projected light beams through the plastic roll and onto an array of photocells, which in turn controlled the pitch of the oscillators. The undulating shapes cut into the paper rolls of the Kangaroo Pouch machine were now simply painted onto the plastic roll with black ink. The circuitry for this machine was transistorised, lending a stability which could not be achieved with the use of valves. The machine was lost in the 1970’s while being transported from Grainger's home in White Plains to the Grainger museum in Melbourne