The LM 567 was a tone decoder chip designed to provide a saturated transistor switch to ground when an input signal is present within the passband.
Originally designed for inexpensive use as Touch Tone decoder, The circuit consists of an I and Q detector driven by a voltage controlled oscillator which determines the center frequency of the decoder.
Ah, google coughs up a datasheet: http://web.mit.edu/6.115/www/datasheets/LM567.pdf
We'll leave the argument as to whether or not the thing is a true PLL for another time, it could be wired up to act like one and so for all intents and purposes I guess it is one.
Here is a Spanish page that has the PCB on it, mentions Popular Electronics magazine, may be the circuit in question here or close to it, not much else and no schematic I could find there: http://py2mg.qsl.br/Filtro%20CW%20PLL/FILTRO%20CW%20PLLtxt.pdf
It appears to have been set up to act more like a bandpass filter than anything else.
I once incorporated the 567 as a 50KHz bandpass filter for a specified ultrasonics design, there was a design kit for this chip that may still be available, you would get two chips, various components and the design and application notes, wouldn't be too hard to set one up as a ~600-800Hz bandpass filter for CW. The thing wouldn't ring, that's likely what the boon would be.