Quartz crystal resonators used in <a href="https://en.wikipedia.org/wiki/crystal_oscillator" class="extiw" title="w:crystal oscillator">crystal oscillators</a>.  Some have their <a href="https://en.wikipedia.org/wiki/resonant_frequency" class="extiw" title="w:resonant frequency">resonant frequency</a> stamped on the case
me
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