" At The outset, these wristwatches have been 1-off or custom creations That always appeared a great deal just like the pocket watches from which they evolved. But shortly, wristwatches took on their own aesthetic, with their own individual technical innovations to unravel new troubles introduced by wrist-borne timekeepers. Weiss https://www.watchesworld.com/a-history-of-watch-reference-numbers/