Serial numbers on Lecoultre watches are generally located either on the back of the watch or inside the watch case on the movement. The numbers tend to be quite small, and may require a magnifying glass or loupe in order to be read.
Lecoultre was founded in 1833 by Antoine Lecoultre, a watchmaker who had invented a new method of manufacturing the pinions that keep time inside the watch. Over the following decades, Lecoultre made several important advances in watchmaking technology, including the invention of the micron that allowed watches to keep time more accurately, and a keyless watch that did not require winding. In 1937, Lecoultre merged with the Jaeger watch company and continues to make watches under the Jaeger-Lecoultre brand as of 2015.