Triple Filtration
I had a later 550.
The triple filtration consisted of:
1. single walled dustbag (reusable)
2. felt filter, pressed into position (later filters had integral air-freshener pocket)
3. fine gauze sealed into a cage. Phillips screw in centre, to lock in place.
The weak point in this style of cleaner was the motor; particularly the belt drive spindle, which wore down at an alarming rate, causing the belt to ride across and cut into the plastic casing. Happened to mine twice, and a few other people I spoke to. Not as durable as the Hoover cleaners.
The Hoover motors of the time could be totally disassembled if you needed to service; the Electrolux on the other hand had a sealed motor.