Some Naim preamps and integrateds need a link plug to be fitted to them if the preamp is connected to a power amplifier and there is no power supply in the system.
The link plugs were supplied with the amplifiers and integrateds when they left the Naim factory but sometimes they get lost.
This is a suitable replacement for the ring locking style of link plug.
If a Naim preamp or integrated is used without a link plug, and no power supply, the preamp won't work properly without the link plug being fitted.
Full details are in the Naim User Manual. You can find a selection of Naim User manuals here