Practicing bottom dealing “with the assistance of a loudly ticking clock or a metronome” was advised as early as 1934 by Graham Adams in Wilford Hutchinson's Conjurers Chronicle, Vol. 15 No. 6, Mar./Apr. 1934, p. 935.
Earlier, a metronome was recommended for practicing silent codes, in which the length of a pause or action transmits information to a partner. See Albert Hopkins's Magic : Stage Illusions and Scientific Diversions Including Trick Photography, 1897, p. 198.