Yes, that is a lighting choke (Very Hi Z at AM RF frequencies) and the tower is insulated from ground and fed through the cap. The Lighting choke allows the 110V (neutral & 2 hot's one flashing and one steady) to cross the base insulator with minimal change in base Z, & radiation. That is an arc gap to give lightning someplace else to go rather than through the cap and other circuits, which it can and WILL do.
As for the Flux cap... you're close, it's a vacuum variable (read very expensive) capacitor.