When an antique clock breaks, a clock that's been telling time for 200 or 300 years, fixing it can be a real puzzle. An old clock like that was handmade by someone. It might tick away the time with a pendulum, with a spring, with a pulley system. It might have bells that are supposed to strike the hour, or a bird that's meant to pop out and cuckoo at you. There can be hundreds of tiny, individual pieces, each of which needs to interact with the others precisely.
To make the job even trickier, you often can't tell what's been done to a click over hundreds of years. Maybe there's damage that was never fixed, or fixed poorly. Sometimes, entire portions of the original clockwork are missing, but you can't know for sure because there are rarely diagrams of what the clock's supposed to look like. A clock that old doesn't come with a manual.
So instead, the few people left in the world who know how to do this kind of thing rely on what are often called "witness marks" to guide their way. A witness mark could be a small dent, a hole that once held a screw. These are actual impressions, outlines, discolorations left inside the clock of pieces that might have once been there. They're clues to what was in the clockmaker's mind when he first created the thing.
I'm told fixing an old clock can be maddening. You're constantly wondering if you've just spent hours going down a path that will likely take you nowhere, and all you've got are these vague witness marks, which might not even mean what you think they mean. So at every moment along the way, you have to decide if you're wasting your time or not.