What is the function of the external resistor used with the 21CYP22?

Prevents excessive arc current, I believe.

^indeed

Where are the two connections on the CRT connected? I would imagine that the connection nearest the electron gun would go to the accelerating element in the electron gun. Where would the connection nearest the screen go? Would it just be connected to the aquadag thus forming a capacitor. If that's the case we, would have a pi network with two capacitors and a resistor in the middle.