I'm not sure how Rex interpreted Grainger’s use of the foot pedals — By ear I would guess
There's not as much information in the 2L booklet as I remembered, so I must have read this too - Duo-Art Reproducing Piano - The Pianola Institute.
For the original recording, Grainger used a Duo-Art reproducing piano which punched holes in paper rolls. Editing was done with razor blades and sticky tape, just like my early computer programming - this was cut and paste for real!
By 1921 recordings were made on full 88-key instruments with the roll running at constant speed, Dynamics and rubato were encoded into the rolls, which had four 'bits' reserved for volume. This theoretically allowed 16 volume levels and was probably set by a second person, usually the producer, using pedals or dials at the time of recording. Because the volume holes controlled analoge vacuum, which was also affected by the number of notes being played and their duration, much greater dynamic variation could be achieved.
Percy Grainger was heavily involved in editing the rolls. He observed that "the Duo-Art represented him not as he actually played, but as he would like to have played".
Originally, the orchestral parts were added to the rolls, and these had to be removed, hole by hole, with sticky tape.
For 2L's recording, a major issue was synchronising the orchestral entry at the end of each piano solo. Microprocessor control allowed the roll to stop after each solo, and the pianolist to restart using a remote control.
So Rex Lawson did not have to interpret for the Piano Concerto, though he did for earlier rolls made by Grieg included on this recording
