The very earliest form of Econet terminator was in fact part of the Econet interface on the BBC motherboard. Up to issue 3, the Econet interface circuitry included the necessary resistors to correctly terminate the network.

This is a picture of my Issue 2 BBC Model B which happens to have the Econet upgrade fitted, although not the clock and termination circuitry which was available on this issue. I'm not quite sure where the terminating components are, although there are some gaps for additional resistors visible just to the right of the Econet socket.

This system meant that a BBC containing the terminator had to be located at one end of the network, which would make the networking system rather inflexible, and so original, mains powered terminators were introduced.