Post 10226
is a Model XXX with an after-market bag converter to allow use of, yes, standard disposable bags with the XXX. You remove the front cover and cloth bag, then the bag converter slips into the round opening and is held in place with two little levers. You open the cover of the bag converter by the two large thumb screws and the cover comes off, then in goes the bag.
I'll post a series of photos showing the bag converter. This was btw a genuine Electrolux aftermarket product that came out some time in the mid 1950s, probably the same time that the Model E came out. They are pretty scarce -- for whatever reason, not very many XXXs were converted.