I hope my age does not interfere with understanding your question.
The
switchbox is attached to the end of the ZX-81 output RF cable, switched to "Computer" with the 300 ohm switchbox twinlead output connected to a 300 to 75 ohm balun.
That balun is connected to the RF input of the device (VCR, TV, Capture card) to see the ZX-81 output.