Eighteen teeth, one drum
The comb is a single steel plate cut into eighteen teeth, each filed to a pitch. The long bass teeth carry small lead weights underneath, which lower them further than length alone could.
The drum turns once in about ten seconds. Every pin is a note at a moment: where it sits along the drum picks the tooth, and how far round it sits picks the time.
Why it keeps time
A spring would let the drum race. The governor spins a small two-blade fan on a worm, and the air it pushes against holds the tempo steady from the first bar to the last.