A usually metal clamshell case for a mechanical

watch A watch is a timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another type of ...

, common until the early twentieth century. It is meant to sit around the inner case of the watch. The term refers to the component holding the main watch components.https://www.wixonjewelers.com/education-type/watch-diagram/

References

Watches Containers {{Fashion-stub