This is used to transmit ethernet/network signals via a house mains AC wiring. They are normally used in pairs, but the other one was taken away after some maintenance leaving just the one. So if you have one like it, you could make a pair with this one.
Collection Oakdale/Fleetsbridge area of Poole.
Ethernet across mains adaptor (Stanley Green BH15)
This offer has been gifted to someone and is no longer available.