This is a DC23 aftermarket post motor hepa filter that fits behind the washable filter. There are 4 phillips screws that need removing to get to this filter. 

This filter should be replaced every 5 years

This filter should be replaced when the motor is replaced

Fits Dyson Models

DC23 serial numbers starting with 

462 469 631 632 633 634 635 771 BB6 BB7 BB8 BB9 CU1 468

Supplying parts to Australia