RCA vintage transformers as a passive summing box - need wiring HELP!

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I have a pair of these I'm going to pull from some old RCA monitor amplifiers, and convert into a passive stereo box just to send through for some color. I just am not quite sure how to read the diagram.

Am I correct in thinking that the in is 3, and the out should be 10, for the 600ohm?... but where is the ground?
Also, what effect would 150 have instead? I read somewhere some people wire switches for different tone using each ohm.

Also, where is the ground wired too?