Further to the above, and as an 'oldie', I seem to recall that the term "Iso Echo" applied to contour was a sort of Ecko Trade Mark for contour circuitry i.e. the inversion of video at a pre-determined level, in trying to verify this found this nice site !
Ecko Radar Systems
In particular the description of the indicator (HS748, BAC 1-11 for certain) mentions it.