A film produced by the NPL Film Unit in the 1950s explaining the principles behind the first accurate atomic clock, designed by Louis Essen and built at the National Physical Laboratory in 1955.
I notice they were talking about achieving good vacuum while showing someone handling a vacuum component with their bare hands, which you wouldn’t normally do — fingerprints outgas. But it was the oven, so all of that junk gets baked off pretty quickly.
The mention a performance of a ten-thousandth of a million, or a part in 10^10. Clocks/frequency standards in use today (i.e. part of time scales, reporting their values) do ~100,000 times better, and experimental ones do even better than that, albeit for relatively short durations.
That device, which is a “physics package” (all the fun stuff) plus 6 equipment racks, now fits into about 6″ of space in a single equipment rack … and the devices are orders of magnitude better.