This is the underneath of the Master ET. None of the connectors are present, so this machine could not be used to connect to disc drives, printers, or 1 MHz devices or external second processors.

It appears a bit strange that the auxiliary power connector is present given that nothing can be connected to this machine, but I imagine that this more to do with the fact that it was probably easier to make each Master with the same PSU rather than having ones with and without the power output.

I do recall seeing a "printer server" version of the Master ET, with a printer port underneath. I can't find any information about such a variant, though, so it's possible that the one I recall had a user modification.

Photograph courtesy of Ken Hewson.