A Roman Arabic dial mixes Roman numerals and Arabic numerals on the same dial. It is sometimes also known as a California dial or Error Proof dial.
Rolex originated the mixed Roman/Arabic dial in the 1940s, and filed a patent on the design in 1941. Their watches used an inverted triangle at 12 and Roman numerals I, II, X, and XI above Arabic numerals 4, 5, 7, and 8. Bar markers were used at 3, 6, and 9. These were part of the so-called “Error Proof” radium dials offered on early Oyster Perpetual models. For this reason, many Rolex fans refer to these as “Error Proof” dials. Rolex also referred to this design as “high visibility”.
In 1944, a similar dial appeared on a Rolex-made watch from Panerai. Although not labeled with either brand's logo, the Panerai 3646 is often associated with Panerai. The modern Panerai company incorrectly dates this to 1936, causing much confusion.
In the 1980s, a dial maker in California began producing replacement Roman Arabic dials for the Rolex bubble back watch. These came to be called “California” dials.