Richard Clark wrote:
"I presume it is the value at the antenna connections of the amplifier
that transforms through the plate tuning toward the plate itself to
render something more like Rp (several KOhms?"

Rp has nothing to do with a Class C amplifier which acts like a switch
presenting an impedance to average current flow by being open-circuit
most of the time.

Best regards, Richard Harrison, KB5WZI