I've been searching for days and still have not come up with an answer.
What is this symbol?:
The component in question was found on this mosfet amplifier schematic from the 1980s.
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This looks like preset potentiometer wired as rheostat. Here you can find the following definitions:
Long story short: It's a variable resistor. Mainly used as a preset resistor (e.g. to equalize an DC offset)
See Rapidtables as an example and search for "Trimmer Resistor" after maybe 1/3 of the page.