Polystyrene capacitors

Those were (and still are, I think) used for high stability for tuned radio circuits. If the circuit timing isn't critical enough to justify a crystal oscillator, you'd probably get by with metallized polyester capacitors, or even epoxy encapsulated ceramic.

Depending on the application, possibly almost anything. I'll bet the author simply had some in his parts bin and used them. They wre pretty popular back then.

Graham

Somewhere, I remember reading that the tempco of polystyrene caps complements that of pot-cores.

That wouldn't surprise me one bit since you often used to find both in things like telecoms filters.

Graham

OK, thanks.

such

be

Probably any modern film cap. Look at the Panasonic ECHU series, for example.