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It's just temperature. With the proper temperature compensation components it shouldn't drift so much that you ever have to retune. So that -750 part may be important after all.
Some engineers would cheat by adding a transformer to keep the oscillator tube warm but that only reduces drift caused by tube temperature variation. Another approach is to make the whole thing a temperature controlled oven, with a thermostat and heater/resistor so that there is essentially no warmup drift. Or there is the compensation approach. None of these is ideal, as they only reduce drift. It's impossible to eliminate it, short of synchronizing with an external or atomic standard.
Some modern radios use a synthesizer, with an accurate crystal oscillator as the standard. My ham radio transceivers work that way; they drift less than 0.5 ppm, something not achievable with an LC oscillator.
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