https://www.typography.com/fonts/decimal/design-notes

Watch lettering is printed through tampography, a technique in which ink is transferred first from an engraved plate to a spongy, dumpling-shaped silicone pad, and from there onto the convex dial of a watch. To reproduce clearly, a letterform needs to overcome the natural tendencies of liquid ink or enamel held in suspension: tiny serifs at the ends of strokes can create a larger coastline, to help prevent liquid from withdrawing due to surface tension; wide apexes on characters like 4 and A eliminate the acute angles where liquid tends to pool. In the two watches above, an Omega 30T2 (ref. 2186) and a Universal Genève Aero-Compax (ref. 22414), artists have taken different liberties with the figures 3 and 7, using different approaches to maximize the openness of these forms. But the peculiar figure 4 is identical, with a low crossbar and a wide apex designed to dilate the counter, defending clarity while producing a silhouette that balances comfortably the other numbers on the dial. How is it that two unrelated watches have the same figure four? How do nearly all vintage watches have it?