Way back in 1960 something Braun in Germany made a little alarm clock
using a 4.194304 MHz crystal which could reach this level of accuracy.
I had one and it certainly met the Harrison level of timekeeping when I
used it for navigation. I think that the crystal cut used had a
temperature coefficient at room temperatures that had minimal variation
at around this frequency. Way better than the 32+ kHz crystals used now.
The clock died in the end but I still have the crystal somewhere :^)
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