Potentiometer R5 (100K) on the meter board adjusts only the RF OUTPUT meter indication. That's not a "real wattmeter" at all, it's really a "relative RF output" meter which should have been calibrated at the factory to read correctly with a perfect 50 Ohm dummy load termination. If the load impedance changes and isn't 50 Ohms, the meter reading will be "off," and could be quite a bit off. And of course the circuit may have drifted over time, since the AL-80A hasn't been manufactured in many years.
Feel free to adjust R5, it won't affect anything else. But don't expect that meter to ever be accurate unless you're using the amp into a 50 Ohm dummy load, and even then it may change a bit from band to band, as it's not really frequency compensated. The detector used is coupled to the RF output via a pair of 10 pF capacitors in series which I would think would make the detector more sensitive on 10m and less sensitive on 80m/160m. It's really just a "relative output" indicator.
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