The Campagnolo 990 was a low cost model that was a direct development of, and replacement for, the unlamented Campagnolo 980. The older Campagnolo 980, despite being launched in 1980, looked like a 1970s model, with straight lines and sharp edges, and completely without any of the spurious curvaceous styling cues popularised in the aero-craze that swept the derailleur world following Shimano's launch of the 1981 Shimano Dura-Ace AX.
Some time around 1985, Campagnolo decided that they needed to update the Campagnolo 980's design, and so they changed the shape of one, and only one, component part - the outer pulley cage plate. They added extravagantly curved aero flanges. In functional terms, it is highly unlikely that these flanges reduced the aerodynamic drag or improved the gear change in any way. When it comes to aesthetics, they can only be described as a gruesome abomination. Most particularly, they jar alarmingly with the rectilinear, angular, sharp-edged knuckles and parallelogram plates.
The Campagnolo 990 is such a lazy, ugly, cynical object that you just have to love it.
I believe that this is a late example of a Campagnolo 990, dating from around 1988. Some of its distinguishing features are:
The eagle-eyed amongst you may note certain similarities between some aspects of this derailleur and those of the Gipiemme Exploit, indicating that the Gipiemme Exploit may have been manufactured by Campagnolo.