From a former F-4 flyer-the reason that chaff worked against the old AA missiles is not becase the chaff showed up strongly on radar
or because it produced a particular radar
signature, it was simply because it could produce multiple (in reality hundreds) of radar signals just strong enough to confuse the old radar based guidance systems on SAM-1 and Sam-2 AA missiles. With multiple signals, those guidance systems could not "lock on" to a single
signal and aim for it. Indeed, such ECM contributed to AA missiles that locked in on engine exhaust
heat rather than a radar signal.
The chaff produced something that would look like modern digital radar seeing a flock of birds-a lot of radar chatter but you can't pick out an individual bird.