The Acorn Electron was the first of several Acorn computers marketed primarily as a "home" computer. As such, the ability to link up to the Econet system probably wasn't one of Acorn's chief concerns when designing the BBC Micro's little brother. I'm pretty sure there was never an official Acorn interface for the Electron. These pictures show an Electron Econet interface manufactured, and probably only available, in Australia by Barson Computers.
The top picture shows the board itself, which on the face of it looks to be a slightly curious shape. However, as the second picture shows, the board plugs into one of the ROM cartridge sockets on an Electron Plus 1 expansion unit.
All the familiar Econet interface components are present: 68B54, Econet ROM, DIN socket, station ID jumpers, and other bits of logic circuitry.
I am grateful to Philip Mulrane for supplying these photos of the Barson Econet interface.