What feature of a watch indicates the presence of double rollers?

The presence of double rollers in a watch is indicated by a feature that includes one ruby pin. This specific characteristic is related to the escapement mechanism in mechanical watches. In horology, double rollers are often associated with the design that improves the reliability and accuracy of the watch's timekeeping. The ruby pin serves as a pivot point for the escape wheel and helps regulate the energy transfer from the mainspring to the escapement, allowing for more precise movement. Thus, the feature of having one ruby pin directly relates to the function of double rollers in enhancing movement efficiency and stability in the watch mechanism.

The other choices do not relate to the specific functionality or mechanics of double rollers in watches. The mechanism for faster winding, a unique time display, and enhancements for water resistance pertain to different aspects of watch design and functionality, rather than the specific presence and role of double rollers.