It's called the California dial. You could wiki it for the full story.... or you could take my word for it that it has something to do with watches worn by soldiers in WW2. Certain countries' soldiers were inclined to wear watches with roman numerals, while others had arabic. To make it more ambiguous to the enemy, they started making half/half dials like these.
Trickery indeed.
here why we call them california dial ,no mistery at all....
The dial was patented by Rolex in 1941 and used by them on manual and automatic watches for about 7 or 8 years. It was known by Rolex as the 'High visibility" dial, as it used quite large luminous numbers. In the 1970s, when the vintage watch boom started, a LA based dial refinisher, Kirk Rich, did some dials with this design. They became popular & many vintage watch stores in California sent their dials to KR. So dealers in the rest of the US & the rest of the world began to call them the 'California' dial, that s all