serve the same purpose which is to align the top 210 and base 220 during compression formation 360 is a part annular an incomplete annulus sleeve which extends around the axis of the compressor and has an opening 361 to accommodate the observation window 224 formation 262 is a complementary part annular channel extending downwardly from the top face 252 of the base 220 the sleeve 360 and the channel 362 are dimensioned such that the sleeve engages the channel prior to any initial compression of the syringeless filtration device 100 strengthening ribs 363 spaced around the sleeve 360 and running axially along the sleeve add rigidity to the sleeve to inhibit bending in use and also increase the effective width of the sleeve to fit more snugly in the channel 362 as the sleeve slides axially in the channel in use without requiring excess use of plastics material as with the collar 260 the sleeve 360 generally blocks the path for any debris if a filter device were to shatter this second embodiment also includes four dimples 270 only one of which is referenced which act as final stops to prevent over compression of the filter device and also act as final alignment members fig6 shows a third embodiment of a compressor 400 which has the same function.