[Electronic M]

I believe you need to obtain an additional separate audio output transformer to build that circuit. I can't see any way the set's own audio output transformer could be connected in that manner while still using it to drive the speaker with any sort of remotely acceptable audio.

I think the pot you are adding is the 100K on the secondary of the audio transformer (which would be the tube side if it were being used for audio). I think the pot on the right is part of the original brightness(?) control circuit. I believe the dashed line on the right pot is an original wire you are supposed to cut so you can wire the new pot/transformer blanking circuit in series.

Hope that makes sense.