I'm more used to seeing CRTs with just one "second anode", painted in aquadag over the inside of the glass bell. It looks like in those ceramic tubes, Tektronix used multiple post-deflection electrodes. What were they for and where is that theory documented? Thanks.
It's "travelling-wave" construction. It's an array of deflection plates mounted and spaced that the beam, in passing between the successive plates, receives an additional amount of deflection. This effectively enables the construction of high bandwidth CRT's. Similar in function to how distributed amplifiers work.
That's not what he's talking about, though that is the other important major feature in the design of these tubes!
The multiple anodes shape the accelerating voltage, changing the scale of the image (potentially distorting it, too, so this must be done carefully), optimizing the deflection sensitivity and image intensity.
The older tubes, that use a painted-on spiral, achieve a gradient field. The spiral of course is a resistor, so the 2nd anode always dissipates some power, even at zero beam current.
Some other CRT designs used multiple 2nd anode voltages, for improving secondary emission (in storage tubes, cameras), or I forget why but Trinitrons used a thing like that too, etc.
Tim