We have found these filter failures more common in instruments that have been switched off for extended periods or that have been previously running on 115V. On quite a few occasions we have had older units back for routine calibration from 115V areas and the filters have failed within minutes of connecting to 240V. Presumably the caps deteriorate but don't actually fail in a way you'd notice. Then having 240 volts across them finishes them off.
It was interesting to see Peter's pictures showing other affected Schaffner filters. Most Solartron equipment (including current production) use Schaffner EMI filters and failure in older equipment is indeed not limited to the FN372-2-21.
For anyone interested a pdf datasheet for the filter can be found here:-
http://www.schaffner.com/en/products...ry-module.html