http://www.boxcarcabin.com/rcaschematics.html
All of these sets have it. Notice how it is implemented. Let's look at the CTC-11 as an example. The voltage drop over R129 is used to bias the vertical deflection yoke. If you look at the vertical output transformer, the resistor is between the GRN and GRN-WHT secondaries, making one a different DC voltage than the other, but both have the same AC voltage. Now, this configuration won't work for the CTC-38, because of the pincushion transformer, and also the vertical output transformer which aren't designed to take DC.
I am thinking of another idea - for the CTC-38. I would much rather use an electronic solution than magnets. I will draw it out when I have a few minutes.
As soon as Electronic M said it is just a DC bias voltage I kind of had a Homer Simpson type moment and realized how this works. What threw me off is the CT-100 schematic, and how that TV achieves vertical centering... it looks a lot more complex than it is!