"California dial" is a watch dial featuring Roman hour markers on the top half and Arabic markers on the bottom half of the dial.
While the origin of the name is unknown, the style of the dial has been around from the early 1930s.
The earliest example I've come across was on a Rolex Oyster bubbleback, from 1938, and the latest dates from 1966, fitted to an Oyster Precision Ref 6424.
The style and logotype itself are typical of the Art Deco period: bold and provocative, yet perfectly balanced.
Nowadays, California dial, is a hallmark of Panerai Radiomir watches, especially models which pay 'tribute' to the original 1938 Radiomir Brevettato Ref 3646.
Here is a sketch of Panerai PAM 249 a highly collectable and sought after piece by any serious Panerai collector.