Face of a <a href="https://en.wikipedia.org/wiki/Riefler_clock" class="extiw" title="en:Riefler clock">Riefler astronomical regulator clock</a>, made by the Clemens Riefler Co. in Munich Anno 1909.  One of the first clocks to have a low thermal expansion <a href="https://en.wikipedia.org/wiki/invar" class="extiw" title="en:invar">invar</a> pendulum rod (not pictured), this was probably the most accurate clock in the world around the turn of the century. It was typically accurate to 15 ms per day, and was guaranteed to be within 20 ms.  Like many precision regulators, the gear train was made with as few gears as possible to reduce friction-induced error.  As a consequence the minute and hour hands are not mounted at the center of the dial but offset on the ends of their driving gears.  The small moon-shaped hole in the face allowed the action of the electric <a href="https://en.wikipedia.org/wiki/remontoir" class="extiw" title="en:remontoir">remontoir</a> which drove the clock to be viewed.
<a href="//commons.wikimedia.org/wiki/User:Mattes" title="User:Mattes">User:Mattes</a>
<span class="int-own-work">Self-photographed</span>