There are two different Superposing Units, but you don't actually need them for the exchange. Their purpose is to "piggyback" additional circuits onto existing lines (e.g. Fullerphone or Teleprinter which are DC telegraphy onto a telephone circuit (all AC) without mutual interference) or to create "Phantom" circuits when there are not enough lines for the expected traffic.
Superposing Unit 1 Transformer is the illustrated cube, and lets you create an earth return phantom circuit or piggyback a Fullerphone onto a speech pair. With more of these (and more pairs available) you can create phantom and "super phantom" circuits. I think it's all in the manual (which is not to hand right now due to the impending house move).
Superposing Unit 3 Transformer is a rectangular box that fits on top of the 10 line U.C. switchboard. It has a fan-out of wires to connect it to the switchboard, and two 10-way sockets to connect the whole assembly to "Box, Terminal, Signal Office", possibly by way of "Frames, D&P, 10 wire" if required. (D&P is Distribution & Protection, and it contains fuses and lightning protectors for each line, plus jumpers to allow rerouting in the event of faults, and hooks to allow more Superposing Units (1 Transformer) to be added to the setup.).
Chris.
(I'm still looking for the 3-transformer superposing unit, the Box, Terminal, Signal Office, and the connecting cables to link it all together; just not right now!)
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