Clock fit ups save time for clockmakers who intend to build high quality wrist watches without having to pick parts from scrape and assemble them. Clock fit ups are also called inserts, and also the terms refer to a manufacturer-preassembled system that gets pushed well right into an opening that is normally pierced in some kind of case. This technique is particularly attractive if one intends the structure to be the centerpiece as opposed to the watch itself. https://www.clockparts.com/clock-inserts-fit-ups/