I saw your PM and am working on the schematic. My box is close to PM capacity and I'd prefer to limit it's use to transactions and confidential info. All you gotta do is ask here and I'll post better pics as soon as I can. [edit: here are the pics]
In the top right hand corner of my third pic you can see the complete G2/focus control circuit. Each color gun has it's own G2 pot the pots connect to the 270V B+ rail and the 720V B+ Boost rail there is no master G2 or anything that would create that effect electrically. The focus is controlled by connecting the black wire from pin 9 of the CRT to one of three points 720V B+ Boost (through R206 for safety), 270V B+ (through R226 also for safety), or directly to chassis ground...You try each connection for focus and leave it set to the one that looks best....You could add a 5M ohm pot between any of the two taps and connect the focus grid of the CRT to the pot's wiper.
I have over 20 deltagun color TVs and have had many more. I can tell you that almost* no deltagun CRT based set ever had a master G2, and yours definitely does not. Master G2 controls did not become standard until inline-gun tubes took over.
* I have a late 60's Admiral that has only a single (master) G2 control...The rest of it's grey scale control system is completely idiotic and ridiculously hard to set up correctly.