C-Scope had their own version of 'off-resonance' detector at the same time. Far better than the normal discrimination of that, even this period but as you say lacking in depth.
You could also keep your eye out for the A.H. Pro models that worked on the same principle.
Another case where they threw the baby out with the bathwater. Modern componants would probably cope better with the drawbacks of the system but all development seemed to stop at the same time.