I'll do some quick research on triode/pentode. You're right. I'm aware of the terms but not really the relevance.
Ok, single heater, single cathode, two sets of (differing) grids, two anodes.
The tube is a triode and pentode in one. The C2 couples the output of the triode plate to the grid of the pentode for output.
AHA!! I can see that! Thanks, Rich!
So, is coupling a term for caps that connect portions of the circuit where something is actively altering things... signal coming in, signal being amplified... but not simply filtering DC or frequency?