The Atom Econet card is a long thin strip which fits underneath the Atom circuit board by connecting to SKT8 on the Atom circuit board.
This is an issue 2 board which has a different layout and different board shape. This particular example has the 6854 chip missing. There are also spaces for adding the clock circuitry to IC6 and IC7 as with the System card, and gaps for resistors, capacitors and diodes for the termination circuitry.
Again there is no 5-pin DIN socket here. This would have been wired to a lead which would connect to the row of five pins on the right-hand side of the board.
Many thanks to Jules Richardson for this picture of the Acorn Atom Econet card from 2004 which I updated in April 2018 from the original at 800 pixels to 2,048 to coincide with the discovery of two different versions of the Atom board.