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Is it in a small metal can about 1/2″ square?
On the schematic, L300 is part of the feedback loop for the PLL circuit. Not positive but the adjustment seems to be used to set the center of the operating range.
I experimented with it when my first Talking House had an FM’ing problem. We called it a “warble”. It seems to have a lot of latitude in adjustment but if you go to far one way or the other the PLL has problems locking in on the ends of the band.
I ended up putting a trimmer capacitor across the PLL reference crystal to “Net” the frequency and that got rid of the warble.