Q1 & Q2 provide linear frequency operation of IC1 following P1 resistance variation.
Q3 was added in order to obtain a louder click, similar to clockwork metronomes.
A 12V micro battery was used to obtain a higher output power and more compactness.
Rotate P1 fully towards R2, then set R1 to obtain 40 beats per minute (compare with another metronome).
Rotate P1 fully towards R3, then set R4 to obtain 208 beats per minute.
Finally mark the entire scale with the usual metronome steps, as follows: