The convertible had a hard filter in the base but it was more to stop solid debris from a broken dust bag getting into the motor, rather than to filter the air. Electrolux was the first to offer filtered air, in fact it was the first and probably last to do so on a dirty-fan machine (152/160/170). The hard filter in the 500 cleaner was never mentioned in the instruction book, nor was the soft filter in the 330 and 345 cylinder. Again, I believe this was an attempt to give the cleaner a ;limited lifespan, by gradually reducing suction power as time progressed.
