I have a wall clock from the 1980s or early 1990s. Probably 1980s. It has a black box behind/"inside" it which powers the whole thing. It's made by Quartz.
On the black box, it says "HF-QUARTZ 4.19 MHz". What do they mean by "HF" and "4.19 MHz"? Does it use radio signals? Can it be told to keep the time automatically by using radio signals at 4.19 MHz? Is that some kind of time-sync radio frequency?
I found no button to press to activate its "auto-time-keeping" feature, if such a thing exists. I first thought that a little black button might be it, but it doesn't seem to be a button after all. It only has "on/off" (called start/stop) and a little wheel you use to manually set the time, mechanically. And a battery, of course.
Does this mean something completely different? Why mention a number of megahertz at all for a wall clock? What kind of significance does it have? Why does the user need to know that it uses "4.19 MHz" for something? What is that something?