The early clocks are electro-mechanical. Every couple minutes they electrically rewind their springs with a light "clonk", I believe by flipping a small weighted gear over. That's my guess.
Easily tested by disconnecting the battery.
More detail here on the mechanism:
https://forums.pelicanparts.com/porsche-911-technical-forum/1095776-vdo-kienzle-clock-repair.html