[G6Tanuki]

Here's a similar circuit from a mid-60s RSGB Handbook.

Text says "the second Z77 stage is designed to operate either as an untuned buffer amplifier on 1.8Mc/s or as a frequency doubler. The change-over is effected by switching in the inductance L2, in combination with the effective grid-to-earth capacitance of the PA valve this introduces a wide band tuned circuit between the driver valve V2 and the PA stage; the capacitor C13 serves merely as a blocking capacitor to prevent short-circuiting of the grid bias"

L2 is listed as "60 turns of 36SWG pile-wound on 3/8-inch diameter former with dust-iron core".

I guess you could work out what the likely inductance of L2 will be, and then see what frequency it would resonate at with a few tens of pF of stray-capacitance.