factory installed clock

How the OEM clocks in old cars worked... A set of mechanical points on a ratchet mechanism would make a pop noise when winding the clock so it would run about 3 minutes, and pop again. As the daily driver would adjust the clock hands forward or backward to correct time shown, this adjustment would cause the clock to run a tiny bit faster or slower respectively. So the clock is/was being fine tuned by the driver. If you remove the battery cable on a classic that is rarely driven, then adjust the clock every time you use it, you have to know to adjust time forward one time and backward the next to keep it somewhere close to correct and not feck it up.
In my opinion, the vacuum performance gauge is the best option.